- 1
Melt butter in a large stockpot or Dutch oven over medium heat. Add onions and cook, stirring frequently, until deep golden brown and caramelized, about 40 minutes. Stir in garlic and cook until fragrant, about 1 minute.
- 2
Stir in wine, scraping up any browned bits from the bottom of the pot. Stir in beef stock, thyme and bay leaves. Bring to a boil and then reduce heat to a simmer. Cook, stirring occasionally, until soup is slightly reduced, about 15 to 20 minutes. Remove and discard thyme and bay leaves.
- 3
Stir in white wine vinegar and season with salt and pepper.
- 4
Preheat oven to broil. Arrange baguette slices in a single layer on a baking sheet. Place into oven and broil until golden brown, about 1 minute. Flip and broil other side for an additional minute. Set aside.
- 5
Divide soup into oven-safe bowls; place bowls onto a baking sheet. Top with baguette slices to cover as much of the surface of the soup as possible. Sprinkle baguette slices with cheese. Place into oven and toast until cheese is golden brown and melted, about 2 minutes. Serve immediately.
