Ingredients :
- 2 tablespoons olive oil
- 1 tablespoon butter
- 3 pounds onions halved and thinly sliced
- 1/2 teaspoon sugar optional
- Salt and freshly ground black pepper
- 1/4 cup balsamic vinegar
- 6 cups chicken broth
- 2 cups beef broth
- 2 teaspoons fresh thyme minced, or ½ teaspoon dried thyme, plus more for garnish
- 1 bay leaf
- 1 baguette sliced and toasted
- 8 slices Gruyére cheese or Swiss cheese, or 2 cups shredded
Directions :
- In a large Dutch oven over medium heat, melt olive oil and butter together. Add onions, sugar (if using), and ¼ teaspoon salt. Cook until the onions are starting to darken and caramelize, about 30 minutes.
- Stir in vinegar and scrape up the brown bits from the bottom of the pot. Add chicken broth, beef broth, thyme, and bay leaf. Simmer until reduced slightly, about 20 minutes. Season to taste salt and pepper.
- Preheat oven broiler on HIGH. Place broiler-safe soup bowls on a rimmed baking sheet. Divide the soup evenly among the 8 bowls (about 1 cup each). Top each bowl with 1 or 2 baguette slices and cover with one slice of cheese (or ¼ cup shredded).
- Broil until the cheese is hot and bubbly, about 3 to 5 minutes, then remove from oven and garnish with fresh thyme if desired.
