In a large Dutch oven, cook onions in 1/3 cup butter until onions turn a dark golden brown. Add mustard, garlic, and sage. Stir in sherry. Add broth.
In a medium saucepan, melt 1/3 cup butter; stir in flour. Cook and stir the mixture until light brown.
Stir flour mixture into onion mixture. Stir in cream. Bring to boiling. Cook for 1 minute. Season to taste with salt and pepper. Serve soup topped with croutons, Swiss cheese, and Parmesan cheese. Makes 8 to 10 side-dish servings.