# 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 3.5 oz)
11 - 1 cup shredded mozzarella cheese (about 3.5 oz)
12 - 2 teaspoons Dijon mustard
# Cooking Steps:
01 - Melt 2 tablespoons unsalted butter in a large skillet over medium heat. Add thinly sliced onions, sugar, salt, and black pepper. Cook, stirring occasionally, until onions turn golden brown and caramelize, approximately 25 to 30 minutes.
02 - Incorporate fresh or dried thyme and balsamic vinegar into the onions. Cook for an additional 2 minutes until aromatic. Remove from heat and set aside.
03 - Butter one side of each bread slice evenly. Spread Dijon mustard on the unbuttered side of two slices.
04 - On the mustard-coated side, layer half of both shredded Gruyère and mozzarella cheeses. Add a generous spoonful of caramelized onions, then top with the remaining shredded cheeses. Place the other bread slices on top, buttered side facing outward.
05 - Heat a clean skillet or griddle over medium-low heat. Grill sandwiches for 3 to 5 minutes per side, pressing gently and flipping once, until bread is golden and cheese melts thoroughly.
06 - Allow sandwiches to rest for 1 minute before slicing. Serve warm.
