# What You'll Need:
→ Caramelized Onions
01 - 2 tablespoons unsalted butter
02 - 2 large yellow onions, thinly sliced
03 - 1 teaspoon granulated 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
11 - 1 cup shredded mozzarella cheese
12 - 2 teaspoons Dijon mustard
# Cooking Steps:
01 - Melt 2 tablespoons butter in a large skillet over medium heat. Add sliced onions, sugar, salt, and pepper. Cook, stirring occasionally, until onions are golden brown and deeply caramelized, approximately 25 to 30 minutes.
02 - Stir in thyme leaves and balsamic vinegar; continue cooking for an additional 2 minutes until fragrant. Remove from heat and set aside.
03 - Butter one side of each bread slice. Spread Dijon mustard evenly on the unbuttered side of two slices.
04 - Layer half of the Gruyère and mozzarella cheeses on the mustard-coated side of the bread. Add a generous spoonful of caramelized onions over the cheese, then top with remaining cheeses. Place the remaining bread slices on top, buttered side facing outward.
05 - Heat a clean skillet or griddle over medium-low heat. Cook sandwiches for 3 to 5 minutes on each side, pressing gently and flipping once, until the bread is golden brown and the cheese is fully melted.
06 - Allow sandwiches to rest for 1 minute before slicing. Serve immediately while hot.
