In a large pot over medium-high heat, melt the butter. Add in the onions and sauté, stirring often, until softened, about 10 minutes.
4 tbsp unsalted butter, 5 large yellow onions
Reduce heat to medium, stir in the salt and sugar; cook for another 35 minutes, stirring occasionally (every 5 minutes or so), until onions are caramelized and almost syrupy.
1 tsp salt, 1 tsp granulated sugar
Add in the garlic and cook until fragrant, about 1 minute.
2 cloves garlic
Pour in the red wine to deglaze the pot, scraping up any brown bits on the bottom and sides.
1/2 cup red wine
Add in the beef broth, chicken broth, Worcestershire sauce, thyme, and bay leaf.
32 ounces low-sodium beef broth, 24 ounces low-sodium chicken broth, 1 tbsp Worcestershire sauce, 2 sprigs fresh thyme, 1 bay leaf
Gently simmer over medium/medium-low heat for about 20 minutes, stirring occasionally.
Remove and discard the herbs.
Mix in vinegar and taste. Season with additional salt and pepper, if necessary.
1/2 tbsp balsamic vinegar
Reduce heat to low, cover to keep hot while you prepare the bread.
Preheat oven broiler, with rack 6 inches from the heat source. Arrange bread slices on a baking sheet and brush both sides with the olive oil. Broil 2-3 minutes, flipping once, until well toasted on both sides. (NOTE: keep an eye on the bread the entire time to ensure it doesn't burn. Every oven is different.) Remove from heat; leave broiler on.
5 1- inch thick slices of French bread, 2 tbsp extra-virgin olive oil
Arrange oven safe bowls or crocks on a rimmed baking sheet. Fill each bowl 2/3 full with hot soup. Top each bowl with 1 slice toasted bread, and sprinkle with the Swiss, Mozzarella, and Parmesan cheeses.
1 1/2 cups shredded Gruyere Swiss cheese, 1/4 cup shredded Mozzarella cheese, 2 tbsp finely grated Parmesan cheese
Broil until bubbly and golden brown. As it softens, the cheese will fall over the sides of the crock and form a wonderful crusty seal.
Serve and enjoy!
pinch of salt and freshly ground black pepper