Coat a Dutch oven with cooking spray; add margarine. Place over medium-high heat until margarine melts. Add onion; saute 5 minutes or until tender. Add broth and next 3 ingredients. Bring to a boil; cover, reduce heat, and simmer 20 minutes. Stir in sherry.
Ladle soup into individual 2-cup ovenproof bowls; place bowls on a baking sheet. Top each serving with a slice of bread. Sprinkle cheese evenly over bread. Bake at 300° for 10 minutes or until cheese melts.