Caramelized onions and Gruyère meet crisp bread for savory, cheesy comfort. Quick, satisfying, and vegetarian.
# What you'll need:
→ Caramelized Onion Filling
01 - 2 tablespoons unsalted butter
02 - 1 large yellow onion, thinly sliced
03 - 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme
04 - 1/4 teaspoon salt
05 - 1/8 teaspoon freshly ground black pepper
06 - 1 teaspoon balsamic vinegar
→ Sandwich Assembly
07 - 4 slices sourdough or country bread
08 - 1 1/2 cups shredded Gruyère cheese
09 - 2 tablespoons mayonnaise or softened butter, for spreading
10 - 1/2 teaspoon Dijon mustard (optional)
# Method:
01 - In a skillet over medium heat, melt the butter. Add the sliced onion, thyme, salt, and black pepper. Cook, stirring often, until the onions are deeply caramelized and golden brown, approximately 18 to 20 minutes. Stir in the balsamic vinegar and cook for 1 additional minute. Remove from heat and set aside.
02 - Lay out the bread slices. Spread mayonnaise or softened butter on one side of each slice.
03 - On the unbuttered side of two bread slices, if desired, spread a thin layer of Dijon mustard. Top each slice with half of the caramelized onions and half of the shredded Gruyère cheese. Close the sandwiches with the remaining bread slices, making sure the buttered side is facing outward.
04 - Heat a nonstick skillet or griddle over medium-low heat. Place the sandwiches in the skillet and cook until the bread is golden and crisp and the cheese has melted, about 4 to 5 minutes per side. Press gently with a spatula as they cook to ensure an even crust.
05 - Slice the sandwiches and serve immediately while warm.
