Serves 4

Adapted from New York Times Cooking

• 8 tablespoons unsalted butter

• 1 pound (500 grams) yellow or Vidalia onions, peeled, halved and thinly sliced

• Kosher salt

• Black pepper

• 4 teaspoons sherry, red-wine, or white-wine vinegar (optional)

• 8 ounces Gruyere cheese, grated or sliced

• 8 slices bread, cut no wider than ½-inch thick

1. Heat a large skillet over medium-high heat. As the pan is getting hot, add the 2 tablespoons butter. Once the butter has melted, add the onions and season with salt and pepper.

2. Cover and cook, stirring once or twice, until the onions are softened, 3 to 5 minutes. Uncover, reduce the heat to medium-low, and cook, stirring occasionally, until deep golden brown, 20 to 25 minutes. If the onions look dry or like they might burn, add a few tablespoons of water at a time, scraping up any browned bits that are stuck to the bottom of the skillet.

   • If desired, once the onions are done, deglaze the skillet with vinegar and cook until the liquid has evaporated, about 1 minute.

3. Transfer the onions to a medium bowl and season to taste with salt and pepper. Add the cheese to the onions and stir to allow the cheese to melt. Wipe out the skillet, or wash it, if necessary.

4. On your cutting board, put down two slices of bread and scoop half of the cheese-onion mixture onto each one. Top with the remaining slices of bread and press down gently.

5. Heat the skillet over medium heat. As the skillet heats up, melt 1 tablespoon butter. Once the butter is melted, add the sandwiches and cook until the bottoms turn golden brown, 3 to 4 minutes, reducing the heat to prevent the bread from darkening too quickly.

6. Add the remaining 1 tablespoon of butter to the skillet and flip the sandwiches, pressing down, letting them cook until the cheese has fully melted and the bottoms turn golden brown, 3 to 4 minutes.

7. Serve as is or pair with a delicious Creamy Tomato Basil Soup.

8. Enjoy!