Slice all onions 1 cm thick. In a 3 litre saucepan over medium-low heat, saute onions and garlic in butter for 15 minutes or until tender and golden, stirring occasionally. Add broth, consomme, Worcestershire sauce and nutmeg; bring to a boil. Reduce heat; cover and simmer for 30 minutes. Sprnkle 1 tablespoon of Swiss cheese in the bottom of six oven-proof 8-oz. bowls. Ladle hot soup into bowls. Top with a slice of bread. Sprinkle with remaining Swiss cheese and Parmesan cheese if desired. Broil until cheese melts. Serve immediately.