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In a large pot over medium-high heat, melt the butter. Add in the onions and sauté, stirring often, until softened, about 10 minutes.
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.
Add in the garlic and cook until fragrant, about 1 minute.
Pour in the red wine to deglaze the pot, scraping up any brown bits on the bottom and sides.
Add in the beef broth, chicken broth, Worcestershire sauce, thyme, and 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.
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.
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.
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!