Ingredients:

  • 2 large yellow onions, thinly sliced (approx. 400g / 14 oz)
  • 2 tablespoons olive oil (30ml / 1 fl oz)
  • 1 tablespoon unsalted butter (15g / 0.5 oz)
  • 1 teaspoon brown sugar (5g / 0.2 oz)
  • 1/4 teaspoon kosher salt (1.5g / 0.05 oz)
  • 1/4 teaspoon black pepper (1.5g / 0.05 oz)
  • 1 tablespoon balsamic vinegar (15ml / 0.5 fl oz)
  • 4 slices sourdough bread, about 1/2 inch thick
  • 4 tablespoons unsalted butter, softened (60g / 2 oz)
  • 8 ounces Gruyère cheese, shredded (approx. 225g / 8 oz)
  • 2 teaspoons Dijon mustard (10ml / 0.3 fl oz)

Instructions:

  1. Heat olive oil and butter in a large skillet over medium heat. Add sliced onions, brown sugar, salt, and pepper. Cook, stirring occasionally, until the onions are deeply golden brown and softened, about 20 minutes. Deglaze the pan with balsamic vinegar and cook for another minute. Remove from heat and set aside.
  2. Spread softened butter on one side of each slice of sourdough bread.
  3. Spread Dijon mustard on the unbuttered side of two slices of bread. Top with half of the shredded Gruyère cheese, then the caramelized onions, and finally the remaining Gruyère cheese. Top with the remaining slices of bread, buttered-side up.
  4. Heat a large skillet over medium-low heat. Place the sandwiches in the skillet and cook for 2-3 minutes per side, or until the bread is golden brown and the cheese is melted and gooey. Press down gently with a spatula to ensure even browning.
  5. Cut the sandwiches in half and serve immediately. Enjoy one of the best sandwich recipes!