Peel and cut the Spanish onions into fairly thick slices.
Add butter, olive oil, salt, and black pepper to a large pot over high heat. Add the sliced onions and cook, stirring frequently, until deeply caramelized (about 55 minutes).
Once the onions are caramelized and there is fond on the bottom of the pot, deglaze with a splash of balsamic vinegar and a splash of red wine. Let the liquids cook down.
Stir in the bay leaves and beef bouillon paste to add extra flavor.
Dust the onions with the all-purpose flour and stir well to lightly cook off the raw flour taste.
Pour in the beef broth and chicken broth. Bring the soup to a boil, then reduce the heat and let it simmer for about 20 minutes to allow the flavors to marry.
Taste and adjust seasoning with additional salt and pepper if needed.
Ladle the hot soup into oven-safe bowls. Add a layer of shredded Gruyère cheese, top with a couple of baguette slices (broken in half for easier eating), and generously cover with more cheese.
Place the bowls under the broiler until the cheese is melted, golden, and bubbling. Let cool slightly before serving.
Cook it hands-free.
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