- 2 large yellow or sweet onions
- 2 large red onions
- 2 tablespoons butter
- 1 clove garlic, finely minced
- 2 thyme sprigs
- 2 bay leaves
- 1 teaspoon salt or to taste
- ¼ teaspoon freshly ground black pepper
- 5 cups vegetable or mushroom broth
- 3 tablespoons Balsamic vinegar
- 1 loaf small French baguette
- 1 cup shredded Gruyere cheese
- Peel and cut onions in half and then finely slice the halves lengthwise.
- Melt butter in a large soup pot or a Dutch oven on medium heat. Once the butter is melted, add the onions. Add pinch of salt and let the onions sweat for 10 to 12 minutes without stirring.
- Stir and let the onions caramelized until dark golden brown on low heat, stirring occasionally. It will take 45 minutes to an hour.
- Add garlic, bay leaves and thyme and sauté for 30 seconds. Add the broth and season with salt and pepper. Simmer the soup for 15 to 20 minutes.
- Remove the thyme sprigs and bay leaves. Add balsamic vinegar and simmer for 5 more minutes.
- Heat the broiler and place the wire rack on top shelf. Cut four ½ inch thick slices of bread. Place the slices on a baking sheet and broil it until golden brown, about 1 minute.
- Ladle the soup in four oven safe bowls, leaving space on top for slice of bread. Place the bread slice on top with broiled side down and top with cheese. Place soup bowls on a baking sheet and broil it for a minute or until cheese is melted. Serve immediately.