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French Onion Mac and Cheese

A large pot filled with French onion mac and cheese, with a few servings missing and a large serving spoon.
What happens when an unstoppable force meets an immovable object? The same thing that happens when French onion soup meets mac & cheese: glorious mayhem. The crispy caramelized onion and Gruyère topping on this dish is absolutely ridiculous.
MOB Kitchen

Prep 15 mins
Cook 45 mins
Total 1 hr
Entree
American
8 servings
656 kcal

Ingredients 

  • 7 tablespoons (3 1/2 ounces) unsalted butter divided
  • 3 large (1 1/4 lbs) onions thinly sliced
  • Salt
  • 1 cup dry white wine
  • Leaves from a small bunch of thyme
  • 1 pound macaroni
  • 1/4 cup all-purpose flour
  • 3 1/3 cups whole milk
  • 7 ounces extra-mature Cheddar cheese grated
  • 7 ounces Gruyère cheese* grated
  • 1 heaping teaspoon Dijon mustard
  • 3 1/2 ounces breadcrumbs
  • Freshly ground black pepper

Directions 

Caramelize the onions

  • In a large skillet over medium heat, melt 2 1/2 tablespoons butter. Add the onions with a good pinch of salt. Sauté, stirring occasionally and reducing the heat if necessary, until the onions have completely softened and caramelized, 30 minutes.
  • Once the onions are cooked, pour in the wine and add most of the thyme leaves, reserving a handful for garnish. Simmer until the wine has reduced by half, 2 to 4 minutes. The onions can be prepared in advance and kept in the fridge.
  • In a large saucepan of boiling salted water, cook macaroni for 2 minutes less than the package instructions instruct, then drain in a colander.

Make the cheese sauce

  • In a large ovenproof skillet over medium heat, add remaining 4 1/2 tablespoons butter. Tip in the flour and whisk to combine. Whisking constantly, slowly pour the cold milk into the flour mixture, a little at a time, to get a smooth sauce. Simmer the sauce until thickened, about 5 minutes.
  • Stir in Cheddar cheese and half of the Gruyère cheese. Once the cheese has melted, take sauce off the heat, add Dijon mustard, and season with salt and black pepper.
  • Preheat broiler to high heat.
  • Tip the cooked macaroni and caramelized onions into the cheese sauce, reserving a tablespoon of the onions. Mix well, then top with remaining onions, Gruyère cheese, and the breadcrumbs. Scatter over a handful more thyme leaves.
  • Broil until golden brown, 2 to 4 minutes. Serve immediately.

Notes

*What can I substitute for Gruyère?

That all depends on why you're not using Gruyère. If you don't like the taste but still want a second cheese in your dinner, consider something milder. Emmental is close--it has the same lovely meltiness but a milder flavor.
MOB Kitchen themselves suggested that if you just don't want the expense of fancy cheeses, you can use all sharp Cheddar (so 14 ounces/400 grams in total). It won't be as French oniony but it will still be incredible.