- 2 tablespoons unsalted butter
- 5 medium red onions (about 3 pounds), halved and sliced thin
- 2 teaspoons minced fresh thyme
- 1/2 teaspoon dried salt
- 1/4 cup balsamic vinegar
- 6 cups low-sodium chicken broth
- 1 3/4 cups low-sodium beef broth
- 2 bay leaves
- 1 baguette, sliced 1/2-inch thick
- 8 ounces swiss cheese, shredded (2 cups)
Step 1: Melt the butter in a large Dutch oven over medium heat. Add the onions, thyme, and 1/2 teaspoon salt. Cook, stirring occasionally, until the onions are dark and sticky, about 30 minutes.
Step 2: Stir in the vinegar and scrape the browned crust from the pot. Stir in the chicken broth, beef broth, and bay leaves. Bring to a simmer and cook for 20 minutes. Season with salt and pepper to taste and discard the bay leaves.
Step 3: Adjust an oven rack to the middle position and heat the oven to 450 degrees F. Arrange oven-safe soup bowls on a rimmed baking sheet and ladle soup into them. Top each bowl with 2 baguette slices and sprinkle each with 1/4 cup of the cheese. Bake until the cheese has melted, about 5 minutes. Serve hot.