# What You'll Need:
→ Caramelized Onions
01 - 2 tablespoons unsalted butter
02 - 2 large yellow onions, thinly sliced
03 - 1 teaspoon sugar
04 - 1/2 teaspoon salt
05 - 1/4 teaspoon freshly ground black pepper
06 - 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme
07 - 1 tablespoon balsamic vinegar
→ Sandwich
08 - 4 slices sourdough or country-style bread
09 - 2 tablespoons unsalted butter, softened
10 - 1 cup shredded Gruyère cheese (about 100 grams)
11 - 1 cup shredded mozzarella cheese (about 100 grams)
12 - 2 teaspoons Dijon mustard
# How to Make It:
01 - Melt 2 tablespoons of butter in a large skillet over medium heat. Add sliced onions, sugar, salt, and pepper. Cook, stirring occasionally, until onions turn golden brown and caramelize deeply, about 25 to 30 minutes.
02 - Stir in thyme and balsamic vinegar; continue cooking for 2 more minutes until fragrant. Remove from heat and set aside.
03 - Butter one side of each bread slice. Spread Dijon mustard on the unbuttered side of two slices.
04 - Layer half of the Gruyère and mozzarella cheese onto the mustarded side. Top with a generous spoonful of caramelized onions, then add the remaining cheese. Place the other bread slices atop, buttered side facing out.
05 - Heat a clean skillet or griddle over medium-low heat. Cook sandwiches 3 to 5 minutes per side, pressing gently and flipping once, until bread is golden brown and cheese is fully melted.
06 - Allow sandwiches to rest for 1 minute before slicing and serving hot.
