1-First Step: Prepare the OnionsMelt 2 tablespoons unsalted butter in a large skillet over medium-high heat. Add 1 pound yellow onions that you’ve peeled, halved, and thinly sliced, then season with kosher salt and black pepper to taste. Cover the skillet and cook, stirring once or twice, until the onions soften, which takes about 3 to 5 minutes.

2-Second Step: Caramelize the OnionsUncover the skillet and reduce the heat to medium-low. Continue cooking the onions, stirring occasionally, until they turn deep golden brown, about 20 to 25 minutes. If the onions start to dry out or burn, add a few tablespoons of water and scrape up the browned bits from the bottom to keep them moist and flavorful.

3-Third Step: Add Optional FlavorOptionally, deglaze the skillet with 2 teaspoons sherry, red-wine, or white-wine vinegar. Cook until the liquid evaporates, which takes about 1 minute, then transfer the onions to a bowl and season to taste with more salt and pepper if needed. This step brightens the onions’ sweetness and prepares them for mixing.

4-Fourth Step: Assemble the SandwichesStir 4 ounces grated Gruyère cheese into the warm caramelized onions until well combined. Place two slices of bread on a surface, scoop half the cheese-onion mixture onto each slice, and top with the remaining bread slices. Press the sandwiches gently to help them hold together.

5-Fifth Step: Cook the SandwichesIn the same skillet, melt 1 tablespoon unsalted butter over medium heat. Add the sandwiches and cook until the bottoms turn golden brown, about 3 to 4 minutes, adjusting the heat as needed to avoid burning. Add the remaining 1 tablespoon butter, flip the sandwiches, press down gently, and cook until the cheese melts fully and the other sides are golden, another 3 to 4 minutes.

6-Final Step: Serve ImmediatelyRemove the sandwiches from the skillet and let them cool for a minute before cutting. Serve right away for the best gooey texture, and enjoy with sides like a fresh salad. The total preparation time is about 45 minutes, with the onions taking around 30 minutes to caramelize, making this a rewarding yet efficient recipe.