Ingredients:

  • 1 large yellow onion, thinly sliced
  • 1 tablespoon olive oil
  • 1 tablespoon butter
  • 1 teaspoon brown sugar
  • 1/4 teaspoon salt
  • 1/4 teaspoon black pepper
  • 2 slices sourdough bread
  • 2 tablespoons unsalted butter, softened
  • 4 ounces Gruyère cheese, shredded or thinly sliced
  • 1 teaspoon Dijon mustard

Instructions:

  1. Heat olive oil and butter in a large skillet over medium-low heat. Add onions, brown sugar, salt, and pepper. Cook, stirring occasionally, until onions are deeply golden brown and softened (about 30 minutes).
  2. Spread softened butter evenly on one side of each slice of bread.
  3. Place one slice of bread, butter-side down, in the skillet. Spread Dijon mustard evenly over the bread. Top with half of the Gruyère cheese, then the caramelized onions, and finally the remaining Gruyère cheese.
  4. Place the second slice of bread on top of the cheese, butter-side up.
  5. Cook over medium heat, pressing down gently with a spatula, until the bread is golden brown and the cheese is melted and gooey (about 4-5 minutes per side).
  6. Cut the sandwich in half and serve hot.