French Onion Potato Gratin

This French onion potato gratin takes the best parts of French onion soup–melty cheese and caramelized onions–and pairs them with potatoes, mushrooms, garlic, and thyme for one helluva great side dish.

A plate of French onion potato gratin

I feel like Britney Spears. Let me explain. I don’t wear short shorts while someone hovers over me controlling everything I do, say, and eat. Well, on second thought, The One has on occasion ruled our life with an iron fist in a velvet glove. No, I feel like Brit of the very early naughts because I keep thinking, “Oops!..I did it again.” And what I’ve done again is turn to the flavors of French onion soup for inspiration.

First, there was my French Onion Skillet Chicken, which is wicked good; if you haven’t made it, I urge you to try it. You’re going to thank me. Then we published French Onion Mac and Cheese−another winner. And, of course, what kind of food site would we be without the mère of them all, classic French Onion Soup. (And if you keep your eyes peeled, you’ll see a few more homages to the French potage in these pages in the coming months.)

A casserole dish of French onion potato gratin

This time I donned my metaphorical red-latex catsuit and headed for the kitchen. There I gave myself the challenge of infusing my favorite potato gratin with sweet caramelized onions, tons of melty Gruyere cheese, and mushrooms–not a classic French-onion ingredient, but, boy, do they add lovely umami notes. Plus the ones on the top of the dish get all marvelously chewy as they brown and sizzle.

Now, you’ll have to tell me if I did indeed do it again, but let me leave you with this: The One has eaten almost this entire gratin by himself and doesn’t feel a bit of guilt. Just saying.

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French Onion Potato Gratin FAQs

What can I substitute for the Gruyère or Emmental in French onion potato gratin?

If you don’t have them on hand, or just want to try something else, you can substitute Gouda or Swiss and get a pretty similar taste and texture. A cheese like fontina will give you a lot of really nice meltiness but will have less flavor.

How do I keep my potatoes from turning gray or brown?

You might find that your sliced potatoes start to darken if they’re sitting for a while. Just dump them into a bowl of cold water until you’re ready to use them and they’ll stay that creamy potato color. Drain and pat dry before using.

A plate and casserole dish of French onion potato gratin

French Onion Potato Gratin

A plate of French onion potato gratin
This gratin takes the best flavors of French onion soupmelty cheese and caramelized onionsand pairs them with potatoes, mushrooms, garlic, and thyme for one helluva great side dish that will have you licking the spoon–even when guests are looking. It's that good!

Prep 25 minutes
Cook 1 hour 15 minutes
Total 2 hours
6 servings
530 kcal
5 / 2 votes
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  • 3 tablespoons (1 1/2 oz) unsalted butter plus more for the dish
  • 10 ounces white or brown mushrooms cleaned and cut into 1/8-inch (3 mm) slices
  • 1 large (8 oz) onion peeled and sliced into thin half-moons
  • Salt and freshly ground black pepper
  • 2 garlic cloves minced
  • 1 tablespoon fresh thyme leaves
  • Pinch of grated nutmeg
  • 1 1/2 pounds (about 4 large) Yukon Gold potatoes
  • 8 ounces grated Gruyère or Emmental cheese
  • 1 1/2 cups heavy cream


  • Position a rack in the middle of the oven and crank the heat to 375°F (190°C). Generously butter a 2-quart (1.9 L) casserole dish.
  • In a large skillet over medium-high heat, melt the butter.
  • Scoop in the mushroom and onion slices and stir to coat. Season with salt and pepper. Continue to cook, stirring occasionally, until the mushrooms and onions are softened and browned, 13 to 15 minutes.
  • Toss in the garlic and thyme and cook until fragrant, 1 to 2 minutes. Stir in the nutmeg. Remove the skillet from the heat.
  • Using a mandoline or sharp knife, carefully cut the potatoes into 1/8-inch (3 mm) thick slices.
  • Line the bottom of the prepared casserole dish with a double layer of potato slices, overlapping slightly—kind of like playing cards. Sprinkle lightly with salt and pepper.
  • Scoop about 1/3 of the mushroom-onion mixture into the dish and spread over the potatoes. It won’t cover it fully, but that’s okay.
  • Sprinkle 1/3 of the cheese over the top.
  • Repeat layering the potatoes, salt and pepper, mushroom-onion mixture, and cheese, two more times.
  • Slowly pour the cream into a crevice along the side, being mindful not to wash away the cheese on top.
  • Slide the casserole into the oven and bake until the potatoes are fork-tender and the top is golden brown, 50 to 60 minutes. If the top browns too quickly, cover it with foil halfway through baking.
  • Remove the casserole from the oven and let sit for 20 minutes before serving.
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Show Nutrition

Serving: 1servingCalories: 530kcal (27%)Carbohydrates: 27g (9%)Protein: 17g (34%)Fat: 41g (63%)Saturated Fat: 25g (156%)Polyunsaturated Fat: 2gMonounsaturated Fat: 12gTrans Fat: 1gCholesterol: 139mg (46%)Sodium: 161mg (7%)Potassium: 771mg (22%)Fiber: 4g (17%)Sugar: 4g (4%)Vitamin A: 1476IU (30%)Vitamin C: 29mg (35%)Calcium: 453mg (45%)Iron: 2mg (11%)

#leitesculinaria on Instagram If you make this recipe, snap a photo and hashtag it #LeitesCulinaria. We’d love to see your creations on Instagram, Facebook, and Twitter.

Recipe Testers’ Reviews

Potato gratins are a family favorite. My mother’s recipe is so good and everyone always wants her to bring them to any family gathering. I would never tell her, but this French onion potato gratin is better!

The flavor of the garlic and onions is present but not overpowering. The thyme really boosts the flavor as well. These are also easy to make. No stirring white sauce and trying to get it the right thickness without overdoing it. Don’t skip the resting time. It helps them “set up” nicely.

They were a great side with a grilled, spatchcocked chicken. The leftovers were delicious the next day as well. 

This French onion potato gratin was a perfect accompaniment to my meatloaf sandwich on sourdough bread. A velvety, creamy sauce with just the right seasoning, and the potatoes were cooked perfectly. I don’t usually gush about potatoes, but these are GOOD—perfect for a weekend supper or potluck.

Not having a mandoline at my disposal, I went for my food processor and its slicing blade which gave the perfect thickness to the potatoes. I suppose I could have done the onions and mushrooms that way as well, but I had the time and sliced them by hand.

This French onion potato gratin is a classic (mostly), delicious, and very rich. The recipe works perfectly as written and the final dish will please nearly anyone, especially when accompanied by a quality grilled steak or roast chicken. 

I might cut the overwhelming richness of the dish a bit next time by using some low-sodium chicken broth in place of at least some of the 1 1/2 cups of cream. 

Still, overall, this is a solid recipe and will undoubtedly grace many a holiday table into the fall and winter months!

This is a great side dish for entertaining. It’s rich and cheesy and would work with most any protein. The addition of the mushroom and onion mixture to the potatoes takes it to the next level. I’m adding this to my Thanksgiving menu this year. I’m also going to make it the day before and wait to bake it Thanksgiving Day.

Let me tell you, this French onion potato gratin is not a casual Wednesday night dinner dish, but it is delicious. I plan on using this dish as a go-to dish when I go to gatherings. I served this with chicken thighs and broccoli.

Originally published November 3, 2021


#leitesculinaria on Instagram If you make this recipe, snap a photo and hashtag it #LeitesCulinaria. We'd love to see your creations on Instagram, Facebook, and Twitter.


  1. 5 stars
    This is a beautiful dish, and a really easy recipe! It has a gorgeous crust, and you’ll want to bring it to the table to show off that, though I find the true treasure is the layers of potatoes and mushrooms coated with cheesy onions. Wait, let me find more elegant phrasing for that that reflects the true character of this dish: onions basking in warm Gruyère. It’s a rich dish, but not a heavy one. That can be a little dangerous, and it’s nicely balanced and like a great pan of brownies, you may find it hard to stop taking just another little bit. And like a pan of brownies, if you are careful with removing your rewards, no one (but you) will know. At least until the pan’s almost done, so if you are serving this for guests and they might be late, you’ll want to make two pans.

    So, that’s what it’s like eating it, and I just consulted with my mother to see what it was like making it. You see, each holiday, we divide up the dishes, and one of our most important holiday traditions is deciding which dishes featured on Leite’s Culinaria we will make. I sent her the 15 Swanky Holiday Side Dishes link, and she chose this one.

    First a character note: While my mom is a very good cook, she is very much a recipe cook. She needs all the steps laid out clearly, and she is not one to stray from the plan.

    On that note, she said this was a “very Eileen-friendly recipe.” She really loved some of the descriptive steps. She especially appreciated, “Line the bottom of the prepared casserole dish with a double layer of potato slices, overlapping slightly—kind of like playing cards.”

    According to Mom, “If you follow this recipe step-by-step, you can’t go wrong.” She sliced the potatoes with a knife, though if she ends up doing this again (or if it’s my turn), I would use or supply (with a finger guard and a lesson) a mandoline. While mom did a great job at slicing these thinly and consistently, if you have a mandoline, do play!

    I look forward to making and having this at holidays and other days again soon! (You definitely don’t have to save this for holidays, and I’ll admit that while it tastes great (hah, I almost typed grate having grated Gruyère on my mind) just slightly cooled from the oven, it is also delicious straight from the fridge. I mean taken from the fridge and served cold on a plate, definitely not eaten from the storage container with the lid underneath as a sort of a cold trivet and the fridge door still slightly ajar. Definitely not like that. And you will find no photo evidence to the contrary.

    1. Stephen, with a review like that, you should become one of our testers! Thank you so much for your incredibly detailed comment and your gorgeous photo. It was like being right there at your holiday table. We love that your mom found this to be easy to follow, and feel incredibly honored that we’re part of your holiday tradition. We can’t wait to hear what ends up on your table next.

    1. Linda, you can. You want to make sure the potatoes are fully covered with the cream, and this will prevent them from turning gray or brown. The easiest way to do this is to cover the whole gratin with plastic wrap and press down the top so it makes contact with the ingredients.

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