Saute onions and mushrooms till soft. Add broth and vinegar. Boil, cover, and simmer for 10 minutes. Heat oven to 400 and heat bread for 10 minutes; remove. Heat broiler. Puree 1/2 the soup mixture; add back to pot, and heat. Divide into 4 broiler proof crocks, top with bread and cheese and broil 3-5 minutes.