Ingredients:

  • 2 large yellow onions, thinly sliced (approx. 400g / 14 oz)
  • 2 tablespoons unsalted butter (28g / 1 oz)
  • 1 tablespoon olive oil (15ml / 0.5 fl oz)
  • 1/2 teaspoon granulated sugar (2g / 0.07 oz)
  • 1/4 teaspoon kosher salt (1.5g / 0.05 oz)
  • 1/4 cup dry sherry or dry white wine (60ml / 2 fl oz)
  • 1 cup beef broth (240ml / 8 fl oz)
  • 1/2 teaspoon fresh thyme leaves, chopped (or 1/4 tsp dried thyme) (1.5g / 0.05 oz)
  • 4 slices sourdough bread (or your favorite crusty bread)
  • 4 tablespoons salted butter, softened (56g / 2 oz)
  • 4 ounces Gruyère cheese, shredded (113g / 4 oz)
  • 2 ounces Swiss cheese, shredded (56g / 2 oz)

Instructions:

  1. Melt butter and olive oil in a large skillet over medium-low heat. Add sliced onions, sugar, and salt. Cook, stirring occasionally, until onions are deeply golden brown and caramelized (about 30 minutes).
  2. Add sherry or white wine to the skillet and scrape up any browned bits from the bottom. Simmer for 2-3 minutes until the alcohol evaporates.
  3. Pour in beef broth and add thyme. Bring to a simmer and cook for 5 minutes, or until the broth has slightly reduced. Remove from heat.
  4. Spread softened butter evenly on one side of each slice of bread.
  5. Place two slices of bread, butter-side down, in a separate bowl. Top with half of the Gruyère cheese, then spoon on the caramelized onion mixture. Top with the Swiss cheese, and finish with the remaining Gruyère cheese.
  6. Place the remaining slices of bread, butter-side up, on top of the cheese. Grill the sandwiches in a skillet over medium heat, pressing down with a spatula, until golden brown and the cheese is melted and gooey (about 3-5 minutes per side).
  7. Cut in half and serve hot.