Ingredients:

  • 1 large yellow onion, thinly sliced (about 2 cups) (220g)
  • 1 tablespoon olive oil (15 ml)
  • 1 tablespoon unsalted butter (14g)
  • 1/4 teaspoon salt (1.5g)
  • 1/4 teaspoon black pepper (0.5g)
  • 1 teaspoon brown sugar (4g)
  • 1 tablespoon balsamic vinegar (15 ml)
  • 2 slices sourdough bread (about 1/2 inch thick)
  • 2 tablespoons unsalted butter, softened (28g)
  • 3 ounces Gruyère cheese, shredded (about 3/4 cup) (85g)
  • 2 tablespoons balsamic glaze (store-bought, or homemade) (30ml)

Instructions:

  1. Sauté onions in olive oil and butter over medium-low heat with salt and pepper, stirring occasionally. Cook until softened and deeply golden brown (about 15-20 minutes). Add brown sugar and balsamic vinegar in the last 5 minutes of cooking; cook until thickened.
  2. Butter one side of each slice of sourdough bread.
  3. Place one slice of bread, butter-side down, in a cold skillet. Top with shredded Gruyère cheese, then the caramelized onions, and finally the remaining Gruyère cheese. Top with the second slice of bread, butter-side up.
  4. Cook over medium-low heat, pressing down gently with a spatula, until the bread is golden brown and the cheese is melted and gooey (about 3-5 minutes per side).
  5. Slice in half diagonally and drizzle with balsamic glaze. Serve immediately.