Ingredients:

  • 4 large onions, thinly sliced
  • 4 tablespoons unsalted butter
  • 1 tablespoon olive oil
  • 2 teaspoons salt
  • 1 teaspoon sugar
  • 8 cups beef broth, preferably low-sodium
  • 1 teaspoon fresh thyme
  • ½ cup dry white wine, optional
  • Freshly ground black pepper, to taste
  • 4 slices of crusty French bread
  • 1 ½ cups shredded Gruyère cheese
  • ¼ cup grated Parmesan cheese, optional

Instructions:

  1. Caramelize the onions: Heat butter and olive oil in a large pot over medium heat. Add onions, salt, and sugar. Sauté for about 40-45 minutes, stirring frequently, until onions are caramelized.
  2. Add liquid ingredients: Add thyme and optional wine. Cook until wine is mostly evaporated, about 5 minutes. Pour in the beef broth and bring to a boil. Reduce heat and simmer uncovered for 15-20 minutes. Season with pepper.
  3. Prepare for serving: Preheat oven to 400°F (200°C). Arrange bread slices on a baking sheet and lightly toast in the oven for 5 minutes, flipping halfway.
  4. Assemble the soup: Ladle the soup into oven-safe bowls. Top each bowl with a slice of toasted bread and sprinkle generously with Gruyère and Parmesan cheese.
  5. Broil to finish: Place bowls on a baking sheet. Broil in the oven until cheese is bubbly and golden, about 3-5 minutes.
  6. Serve and enjoy: Carefully remove bowls from the oven. Let cool slightly before serving.