French Onion Grilled Cheese

1 tablespoon extra-virgin olive oil

2 yellow onions, thinly sliced

Kosher salt and freshly ground black pepper

4 tablespoons unsalted butter, at room temperate

8 slices sourdough bread

1 tablespoon Dijon mustard, plus more for serving

8 ounces Gruyère cheese, shredded

In a large skillet, heat the olive oil over medium-low heat. Add the onions and season with a generous pinch of salt. Cook, stirring frequently, until the onions are deeply caramelized, 30 to 40 minutes. (Keep an eye on the onions and adjust the heat as needed—they should slowly soften before turning golden, but you don’t want any browning or crisping. Add a tablespoon or two of water if they are cooking too fast.) The caramelized onions can be held in the fridge for up to 5 days.
To make the sandwiches, butter one side of each slice of bread, then place butter side down on your work surface. On half of the slices, spread a thin layer of mustard and season with pepper. Divide the cheese evenly between the four slices, add the caramelized onions, then close the sandwiches with the remaining bread.
In a large skillet over medium-low to medium heat, cook the sandwiches until one side is golden brown, about 5 minutes. Flip, then cook the other side until golden brown, about 3 minutes more. (Depending on the size of your skillet, you may need to do this in batches.) Serve the sandwiches with more mustard for dipping.