1-First Step: Mise en place Gather all ingredients and tools: a large skillet, a grater for the Gruyère, a spatula, a cutting board, and a sharp knife. Peel and halve the onions, then thinly slice them. Grate 4 ounces of Gruyère and set aside. Cut your bread so slices are no wider than ½-inch thick. Measure butter and have your vinegar handy if you plan to deglaze.

2-Second Step: Begin caramelizing the onions In a large skillet over medium-high heat, melt 2 tablespoons of unsalted butter. Add the sliced onions and season with a pinch of kosher salt and a grind of black pepper. Cover and cook until the onions soften, about 3 to 5 minutes. This initial steam step helps the onions soften without browning too fast.

3-Third Step: Low-and-slow caramelization Uncover the skillet, reduce the heat to medium-low, and continue cooking the onions while stirring occasionally. Cook until the onions are deep golden brown and jammy, about 20 to 25 minutes. If the onions start to dry or stick, add a splash of water, 1 to 2 tablespoons at a time, to deglaze and control the browning. If desired, add 2 teaspoons of sherry, red-wine, or white-wine vinegar toward the end and cook until the liquid evaporates. Transfer the finished onions to a bowl and taste; adjust salt and pepper as needed.

4-Fourth Step: Combine cheese and onions Stir the grated Gruyère into the warm onions so the cheese softens and mixes evenly. This distributes the cheese through the mixture so every bite contains melted Gruyère alongside sweet caramelized onions.

5-Fifth Step: Assemble the sandwiches Place two slices of bread in a clean area and divide the cheese-onion mix evenly across them. Top with the remaining bread slices and press gently to compact the filling. Keep the assembled sandwiches close to the stove for quick grilling.

6-Sixth Step: Grill the sandwiches Heat a skillet over medium heat and melt 1 tablespoon of unsalted butter. Place the sandwiches in the skillet and cook until the bottoms are golden brown, about 3 to 4 minutes. If the bread browns too quickly, lower the heat to medium-low. Add the remaining tablespoon of butter, flip the sandwiches carefully, press down gently with a spatula, and cook another 3 to 4 minutes until the second side is golden and the cheese is fully melted. If the cheese needs help melting, cover the pan for 30 to 60 seconds to trap steam and speed melting.

7-Final Step: Rest and serve Remove the sandwiches from the pan and let them rest for 1 minute before cutting. This short rest keeps the filling from spilling and lets the melted cheese settle a little. Serve immediately with a simple side salad or a bowl of tomato soup for dipping.