Preparation Preheat oven to 350°F. Bake bread until crisp and cheese begins to melt, about 15 minutes. Melt butter in heavy large pot over medium-high heat. Add onion and fennel; sauté until deep brown around edges, about 10 minutes. Stir in broth and thyme. Simmer, uncovered, until onion and fennel are soft, about 5 minutes. Stir in liqueur, if desired; simmer 2 minutes. Season with salt and pepper. Ladle soup into bowls; place 1 slice bread atop each.
Preparation Preheat oven to 350°F. Bake bread until crisp and cheese begins to melt, about 15 minutes. Melt butter in heavy large pot over medium-high heat. Add onion and fennel; sauté until deep brown around edges, about 10 minutes. Stir in broth and thyme. Simmer, uncovered, until onion and fennel are soft, about 5 minutes. Stir in liqueur, if desired; simmer 2 minutes. Season with salt and pepper. Ladle soup into bowls; place 1 slice bread atop each.