# What You 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
# Steps:
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 are deeply golden and caramelized, approximately 25 to 30 minutes.
02 - Incorporate thyme leaves and balsamic vinegar into the onions. Cook for an additional 2 minutes until fragrant, then 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 - Distribute half the Gruyère and mozzarella cheeses over the mustard-spread sides. Top with a generous portion of caramelized onions, then layer remaining cheeses. Cap with the remaining bread slices, buttered side facing outward.
05 - Preheat a clean skillet or griddle over medium-low heat. Cook sandwiches 3 to 5 minutes per side, pressing gently and flipping once, until bread achieves a golden crust and cheese melts thoroughly.
06 - Allow sandwiches to rest for one minute before slicing. Serve while warm.
