BRING first 5 ingredients to boil in large saucepan on medium heat, stirring occasionally. Simmer on medium-low heat 10 to 15 min. or until peppers and mushrooms are tender, stirring occasionally.
MEANWHILE, heat broiler. Place bread slices on baking sheet; sprinkle evenly with cheese. Broil 3 to 5 inches from heat, 1 to 2 min. or until cheese is melted.
SPOON soup into 6 serving bowls. Serve topped with cheese-topped bread.