Coat a Dutch oven with cooking spray; add margarine. Place over medium heat until margarine melts. Add onion; cook 20 to 25 minutes or until golden, stirring frequently. Add sugar and pepper; stir well. Add flour; cook, stirring constantly, 1 minute. Gradually add beef broth. Bring to a boil; cover, reduce heat, and simmer 30 minutes.
Place 8 ovenproof soup bowls on a baking sheet; ladle soup evenly into bowls. Top each with a slice of French bread. Sprinkle cheeses evenly over bread. Broil 5 1/2 inches from heat (with electric oven door partially opened) 3 minutes or until cheese melts. Serve immediately.