1. In a large skillet over medium heat, melt the butter. Add the sliced onions and sprinkle with sugar. Cook, stirring occasionally, until the onions are soft and golden brown, about 25-30 minutes.
2. Stir in the balsamic vinegar, and season with salt and pepper. Continue to cook for another 5 minutes until the onions are caramelized and fragrant. Set aside.
3. Preheat a large non-stick skillet over medium heat. Spread mayonnaise on one side of each bread slice—this will ensure a golden, crispy crust.
4. Assemble the sandwich: Layer two slices of bread with a slice of Gruyère cheese, a generous portion of caramelized onions, and a sprinkle of mozzarella cheese. Top with the remaining slices of bread, mayonnaise side out.
5. Place the sandwiches in the skillet and cook until the bread is golden brown and the cheese is melted, about 3-4 minutes per side. Press down gently with a spatula to help the cheese melt evenly.
6. Once done, remove the sandwiches from the skillet and let them cool for a minute before slicing. Optionally, garnish with fresh thyme leaves for an aromatic touch.

Making this sandwich, I’m reminded of the patience my abuela had as she taught me how to get that perfect caramelization on onions. It’s a small labor of love that pays off beautifully in flavor.