Challah Grilled Cheese

In a heavy-bottomed skillet over medium-high heat, warm the oil until nearly smoking, about 5 minutes.
Add the onions and cook, stirring often and reducing the heat to if necessary, until translucent, 5 to 6 minutes.
Add the pastrami, stir, and cook for 3 to 4 minutes.
Add the sugar, vinegar, and maple syrup and stir to combine. Bring to a simmer and cook until the liquid begins to thicken, stirring occasionally, 8 to 10 minutes.
Reduce the heat to medium-low and cook until the mixture is thick and dark in color, stirring every 15 minutes or so to prevent sticking and burning, 35 to 55 minutes more. Remove from the heat.
Using a food processor or an immersion blender, process the mixture to a jammy consistency. Transfer to an airtight container and let it cool. (You can cover and refrigerate the pastrami jam for up to 1 week or freeze it for up to 3 months. To return it to a spreadable consistency, thaw and then microwave at 50% power for 2 about minutes to warm it through.)
☞ TESTER TIP: You’ll have a heck of a lot more pastrami jam than you’ll need for a single grilled cheese. This is not a bad thing. Spoon what’s left on any sandwich or burger.

Heat a seasoned cast-iron skillet (a nonstick pan will do in a pinch) over medium heat until hot, 4 to 5 minutes.
Place the bread on a cutting board. Spread 1 1/2 tablespoons jam on a slice of challah. Top it with a slice of American cheese. Add the Gruyère and the second slice of the American cheese. Place the second slice of challah on top of the American cheese and spread the top with half the mayonnaise, smearing it from edge to edge.
Reduce the heat to medium-low. Carefully pick up the sandwich place it, mayonnaise-slathered side face down, in the center of the skillet. Cook the sandwich, undisturbed, until the cheese begins to melt, 4 to 6 minutes.
☞ TESTER TIP: You may need to reduce the heat to low if the bread starts turning dark too quickly.
While the first side cooks, carefully spread the remaining mayonnaise on the top slice of bread. Use a spatula to carefully flip the sandwich. Press down on the sandwich with the spatula and cook until the bottom is golden brown and the cheese has melted, 4 to 5 minutes more.
Remove the grilled cheese from the skillet and transfer to a cutting board. Cut as desired and serve ASAP.