# The Ingredients 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 (approximately 3.5 ounces)
11 - 1 cup shredded mozzarella cheese (approximately 3.5 ounces)
12 - 2 teaspoons Dijon mustard
# Step-by-Step Instructions:
01 - Melt 2 tablespoons unsalted butter in a large skillet over medium heat. Add the sliced onions, sugar, salt, and black pepper. Cook, stirring occasionally, until the onions are golden brown and deeply caramelized, approximately 25 to 30 minutes.
02 - Incorporate thyme leaves and balsamic vinegar into the caramelized onions. Cook for an additional 2 minutes until fragrant, then remove from heat and set aside.
03 - Butter one side of each bread slice evenly. Spread Dijon mustard on the unbuttered sides of two slices.
04 - Layer half of the Gruyère and mozzarella cheeses onto the mustard-spread side of two bread slices. Top with a generous portion of the caramelized onions, then add the remaining cheeses. Place the other bread slices on top with the buttered sides facing outward.
05 - Heat a clean skillet or griddle over medium-low heat. Cook the assembled sandwiches for 3 to 5 minutes per side, pressing gently and flipping once, until the bread is golden brown and the cheese has fully melted.
06 - Allow the sandwiches to rest for 1 minute before slicing. Serve immediately while warm.
