Ingredients:

  • 3 tablespoons unsalted butter (45g)
  • 2 tablespoons olive oil (30ml)
  • 3 large yellow onions, thinly sliced (about 3 lbs/1.4kg)
  • 1 teaspoon granulated sugar (5g)
  • 1/2 teaspoon dried thyme (1g)
  • 1/4 teaspoon black pepper (0.5g)
  • 1/4 teaspoon kosher salt, plus more to taste (1g)
  • 1 tablespoon balsamic vinegar (15ml)
  • 6 cups beef broth (1.4L)
  • 1 cup dry red wine (240ml)
  • 1 tablespoon Worcestershire sauce (15ml)
  • 1 bay leaf
  • 1 baguette, cut into 1-inch thick slices (2.5cm)
  • 8 ounces Gruyère cheese, shredded (225g)
  • Fresh parsley, chopped (for garnish - optional)

Instructions:

  1. Melt butter and olive oil in a large pot over medium heat. Add onions, sugar, thyme, salt, and pepper. Cook, stirring occasionally, until onions are deeply golden brown and softened (about 30-40 minutes). Stir in balsamic vinegar; cook 1 minute. Remove from pot and set aside.
  2. In the same pot, combine beef broth, red wine (if using), Worcestershire sauce, and bay leaf. Bring to a simmer and cook for 15 minutes to allow flavors to meld. Remove bay leaf.
  3. Toast baguette slices in a toaster, under a broiler, or in a skillet until lightly golden.
  4. Arrange half of the baguette slices in a single layer in the baking dish. Top with half of the caramelized onions. Pour half of the beef broth mixture over the bread and onions. Repeat layers with remaining bread, onions, and broth.
  5. Sprinkle shredded Gruyère cheese evenly over the entire casserole.
  6. Bake in a preheated oven at 375°F (190°C) for 30-40 minutes, or until the cheese is melted, bubbly, and golden brown.
  7. Let stand for 5-10 minutes before serving. Garnish with fresh parsley, if desired.