Ingredients:

  • 2 large yellow onions, thinly sliced (US: approx. 1 lb, Metric: 450g)
  • 2 tablespoons olive oil (US: 30ml, Metric: 30ml)
  • 1 tablespoon unsalted butter (US: 14g, Metric: 14g)
  • 1 teaspoon brown sugar (US: 4g, Metric: 4g)
  • 1/4 teaspoon kosher salt (US: 1.5g, Metric: 1.5g)
  • Pinch of black pepper (US: pinch, Metric: pinch)
  • 1 tablespoon balsamic vinegar (US: 15ml, Metric: 15ml)
  • 4 slices sourdough bread, about 1/2 inch thick (US: 4 slices, Metric: 4 slices)
  • 4 ounces Gruyere cheese, shredded or thinly sliced (US: 113g, Metric: 113g)
  • 2 tablespoons unsalted butter, softened (US: 28g, Metric: 28g)

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 20 minutes). Stir in balsamic vinegar during the last minute of cooking. Set aside.
  2. Spread softened butter evenly on one side of each slice of bread.
  3. Place two slices of bread, butter-side down, in a clean skillet. Top with Gruyere cheese, then the caramelized onions, and then more Gruyere. Top with the remaining bread slices, butter-side up.
  4. Cook over medium-low heat until the bread is golden brown and the cheese is melted and gooey (about 5-7 minutes per side). Press down gently with a spatula during cooking to ensure even browning.
  5. Cut in half and serve warm.