French Onion Soup

Name: French Onion Soup
Prep time: 15 min
Cook time: 1 h 10 min
Total time: 1 h 25 min
Cuisine: French
Ingredients: 4 tablespoons unsalted butter; 5 large yellow onions (, thinly sliced); 1 teaspoon salt; 1 teaspoon granulated sugar; 2 cloves garlic (, minced); 1/2 cup red wine; 32 ounces low-sodium beef broth; 24 ounces low-sodium chicken broth; 1 tablespoon Worcestershire sauce; 2 sprigs fresh thyme; 1 bay leaf; 1/2 tablespoon balsamic vinegar; pinch of salt and freshly ground black pepper ( , to taste); 5 1-inch thick slices of French bread; 2 tablespoons extra-virgin olive oil; 1 1/2 cups shredded Gruyere Swiss cheese (, room temperature); 1/4 cup shredded Mozzarella cheese (, room temperature); 2 tablespoons finely grated Parmesan cheese (, room temperature)