Melt margarine in Dutch oven or large saucepan over low heat. Add onions; cook 15 minutes or until onions are golden brown and tender, stirring occasionally.
Add broth, bouillon cube, Worcestershire sauce and pepper. Bring to a boil. Reduce heat; cover and simmer 20 to 25 minutes.
Meanwhile, in medium bowl, combine cheeses.
To serve, place 6 ovenproof bowls on cookie sheet for ease in broiling. Ladle soup into bowls. Top each with slice of toasted bread; sprinkle each with about 2 tablespoons cheese mixture. Broil about 3 to 5 inches from heat for 1 to 3 minutes or until cheese is bubbly.