# What You Need:
→ Caramelized Onions
01 - 2 medium yellow onions, thinly sliced
02 - 2 tablespoons unsalted butter
03 - 1/2 teaspoon kosher salt
04 - 1/4 teaspoon freshly ground black pepper
05 - 1/2 teaspoon sugar (optional)
06 - 1 teaspoon balsamic vinegar (optional)
→ Sandwich
07 - 4 slices rustic sourdough or country bread
08 - 2 tablespoons unsalted butter, softened
09 - 5 ounces Gruyère cheese, grated
10 - Caramelized onions (prepared above)
# Steps:
01 - Heat 2 tablespoons butter in a large skillet over medium-low heat. Add sliced onions, salt, and pepper. Cook, stirring occasionally, for 25 to 30 minutes until onions develop a deep golden color and caramelize. Stir in sugar and balsamic vinegar if using; cook for an additional 2 to 3 minutes. Remove from heat.
02 - Arrange bread slices and spread one side of each with softened butter. Place two slices butter-side down, distribute half the grated Gruyère evenly over them. Spread caramelized onions on the cheese, then top with remaining Gruyère. Close sandwiches with remaining bread slices, butter-side up.
03 - Preheat a nonstick skillet or griddle over medium heat. Place sandwiches in the pan and cook 3 to 4 minutes per side, pressing gently, until the bread is golden brown and the cheese melts. Lower heat if bread browns too quickly before cheese is fully melted.
04 - Slice sandwiches and serve immediately.