• Melt the butter with 1 tablespoon olive oil in a large cast-iron Dutch oven or other heavy-bottomed pot over medium heat. Add the sliced onions and season lightly with salt. Cook, stirring occasionally at first, until the onions begin to release their moisture and soften. As they reduce and begin browning, stir more frequently and scrape the bottom of the pot as needed.

  Continue cooking until the onions are deep golden brown, very soft and jammy, about 50–60 minutes.
• Add the minced garlic and cook for 30–60 seconds, stirring, just until fragrant.
• Pour in ½ cup dry white wine. Use a wooden spoon to scrape up the browned bits from the bottom of the pot. Let the wine simmer until it has reduced substantially and there is very little liquid pooling around the onions.
• Pour in 4 cups beef broth and add the fresh thyme and 2 bay leaves. Bring to a gentle simmer and cook uncovered for 25 minutes, allowing the flavors to develop.
• Remove the thyme stems and bay leaves. Taste and adjust with salt and freshly ground black pepper as needed.

Toasting the Baguette

• While the soup simmers, preheat the oven to 400°F.

  Arrange the baguette slices on a baking sheet and lightly brush them with remaining 1 tbsp olive oil. Bake for about 5 minutes, or until lightly golden and crisp.

Assemble the Soup

• Switch the oven to broil.

  Place oven-safe soup crocks on a rimmed baking sheet. Ladle the hot soup into the bowls and top each with a toasted baguette slice and a generous amount of shredded Gruyère.
• Broil for approximately 2–3 minutes, watching closely, until the cheese is melted, bubbly and golden brown in spots.

  Carefully remove from the oven and serve immediately. The bowls will be very hot.