Melt butter in a large Dutch oven. Add onion; cook over medium heat 20 to 25 minutes or until golden, stirring frequently. Add sugar, salt, and pepper; stir well. Add flour; cook, stirring constantly, 1 minute. Gradually add water, beef broth, and consomme. Bring to a boil; cover, reduce heat, and simmer 30 minutes. Stir in Cognac and Worcestershire sauce.
Place 8 ovenproof soup bowls on a baking sheet; ladle soup evenly into bowls. Top each with a slice of French bread. Place cheese slices evenly over bread, and sprinkle evenly with Parmesan cheese. Broil 5 1/2 inches from heat (with electric oven door partially opened) 5 minutes or until cheese melts. Serve immediately.