Melt butter in heavy-bottomed Dutch oven. Add sliced onions and leeks, garlic, bay leaf and star anise. Cook over medium heat, stirring often, until onions caramelize and brown, about 25 minutes.
Add wine; cook until evaporated. Add stock or broth and Worcestershire sauce. Bring to boil; reduce heat to low and simmer, partially covered, for 45 minutes.
Season soup with salt and pepper to taste. Remove bay leaf. For each serving, ladle 1 cup hot soup into ovenproof crock. Float 1 oz. oven-toasted croutons atop soup. Cover croutons with 1-1/2 oz. shredded BelGioioso Fontina. Broil until cheese melts.