In a large stockpot on medium-high heat, saute the onions with the butter and bay leaf for 20 minutes, until the onions turn a rich golden brown color. Deglaze the pan with the sherry and brandy and simmer uncovered for 5 minutes. Add the white wine and simmer uncovered for 15 more minutes.
Add the beef and chicken stocks plus salt and pepper. Bring to a boil, then simmer uncovered for 20 minutes. Remove the bay leaf, taste for salt and pepper.
*For the croutons*.
While the soup simmers, arrange the baguette slices in single layer on baking sheet and bake in a 400-degree oven until the bread is dry, crisp, and golden at edges, about 10 minutes. Set aside.
Adjust oven rack 6 inches from broiler element and heat broiler. Set individual broiler-safe crocks on baking sheet and fill each with about 1 3/4 cups soup. Top each bowl with 1 or 2 baguette slices (do not overlap slices) and sprinkle evenly with Gruyère. Broil until cheese is melted and bubbly around edges, 3 to 5 minutes. Let cool 5 minutes before serving.