Halve and slice the Spanish onions ¼ inch thick. Remove the tops and cores from the fennel and slice it ¼ inch thick. Remove the crusts from the sourdough or white French boule and slice it ½ inch thick. Grate the Gruyère cheese.

Heat the butter and olive oil in a large stockpot over medium-high heat. Add the onions and fennel, and cook over medium heat for 30 to 40 minutes, stirring occasionally, until the onions turn a golden brown. If the onions aren't browning, turn the heat up. Add the sherry and Cognac, scraping up the brown bits in the pan, and simmer uncovered for 5 minutes. Add the white wine and simmer uncovered for 15 more minutes. Add the beef broth, bay leaves, salt, and pepper, bring to a boil, lower the heat, and simmer uncovered for 20 minutes. Remove the bay leaves and taste for seasoning.

Preheat the broiler and position a rack 5 inches below the heat source. Ladle the soup into heat-proof serving bowls, top with the toasted bread, sprinkle generously with grated Gruyère, and broil for 3 to 5 minutes, until the cheese is melted and bubbly.

Serve the Onion and Fennel Soup Gratin hot.