- 1/4 cup butter
- 2 pounds sweet or yellow onions, or a mix, thinly sliced
- 3 cloves garlic, minced
- 1 teaspoon chopped fresh thyme
- Salt and pepper, to taste
- Splash of balsamic vinegar
- 1 tablespoon all-purpose flour
- 1/4 cup sherry or white wine
- 6 to 8 cups beef or vegetable broth
- 8 (1/2-inch-thick) slices baguette, toasted
- 1 1/2 cups shredded Gruyère cheese
Melt butter in a large, heavy stockpot over medium heat. Add onions and cook, stirring often, until softened, about 15 minutes. Increase the heat to medium-high and cook onions, stirring frequently, until brown, about 10 minutes.
Stir in garlic, thyme, salt, pepper and a generous splash of vinegar; cook for 2 minutes. Sprinkle flour over onions and stir to coat; cook for 2 minutes.
Deglaze the pan with sherry or wine. Pour in stock and bring to a boil. Reduce heat to a simmer and cook for 15 minutes to meld flavors and thicken broth.
Preheat a broiler. Transfer soup to 4 oven-safe ramekins or bowls. Top each soup with 2 baguette slices and sprinkle with cheese. Broil until cheese melts, about 2 minutes. Serve immediately.