Ingredients:

  • 2 large yellow onions, thinly sliced (about 400g)
  • 2 tablespoons olive oil (30ml)
  • 1 tablespoon butter (15g)
  • 1/2 teaspoon salt (2.5g)
  • 1/4 teaspoon black pepper (1.25g)
  • 1 tablespoon balsamic vinegar (15ml)
  • 1 teaspoon brown sugar (5g) (optional)
  • 4 slices sourdough bread (about 1 cm thick)
  • 4 tablespoons unsalted butter, softened (60g), divided
  • 6 ounces (170g) Gruyere cheese, shredded or thinly sliced
  • 2 tablespoons Dijon mustard (30ml)

Instructions:

  1. Heat olive oil and butter in a large skillet over medium-low heat. Add sliced onions, salt, and pepper. Cook, stirring occasionally, until onions are soft and translucent (about 10 minutes). Reduce heat to low and continue cooking, stirring frequently, until onions are deeply golden brown and caramelized (about 20-30 minutes). Stir in balsamic vinegar and brown sugar (if using) during the last 5 minutes of cooking. Remove from heat and set aside.
  2. Spread Dijon mustard evenly on the inside of two slices of sourdough bread.
  3. Divide the Gruyere cheese evenly between the two slices of bread spread with mustard. Top with the caramelized onions, dividing evenly. Top with the remaining two slices of sourdough bread.
  4. Spread softened butter evenly on the outside of both sandwiches. Place sandwiches on a preheated griddle or in a panini press. Grill until golden brown and cheese is melted and gooey, about 3-5 minutes per side. If using a skillet, press down on the sandwiches with a spatula to ensure even browning.
  5. Cut sandwiches in half and serve while warm.