MELT margarine in large skillet on medium heat. Add onions; cook about 15 minutes or until golden brown, stirring frequently. Add broth, water and bay leaf; stir. Cover; simmer 15 minutes. Remove and discard bay leaf.
PREHEAT broiler. Ladle hot soup evenly into 6 ovenproof bowls; top each bowl of soup with a toast slice. Sprinkle evenly with cheeses.
BROIL 5 to 10 minutes or until cheese is golden brown.