Start with the star of the show: the onions. Melt 2 tablespoons of butter in a large skillet over medium-low heat. Once melted, toss in the onions. Stir consistently to ensure even cooking. You want the onions to caramelize slowly, taking about 25-30 minutes. Add a pinch of salt to help bring out the moisture. As they cook, enjoy the aroma that fills your kitchen. These sweet gems will transform from sharp and raw to deep brown and succulent. When they resemble golden perfection, stir in the vinegar and cook for another minute. Finally, set the onions aside in a bowl.

While the onions are cooling, let’s focus on the bread and cheese. Lay out your slices of bread and sprinkle an even amount of grated Gruyère onto two of the slices. Feel free to get a little extra cheesy here. Add that heavenly onion mixture on top of the cheese, then pile more cheese on top. This stacking will create the perfect gooeyness that holds everything together!

Now, layer the remaining bread slices on top of your creations. Spread the remaining butter on the outer sides of the sandwich. This is crucial for getting that perfect crispy finish you desire.

Heat the skillet over medium heat. Place your sandwiches in the pan. Cook until the bread is golden brown, approximately 3-4 minutes per side. Carefully flip only when they have a lovely crust. You want the cheese to melt completely, so be patient if needed.

Once both sides are golden, remove the sandwiches from the heat. Let them sit for a minute before slicing them in half. You want to avoid burning your mouth on that molten cheese. Serve warm and enjoy!