Sauté onion in butter in Dutch oven until tender. Blend in flour, stirring until smooth. Gradually add chicken broth, beef broth, water, and wine. Bring to a boil; reduce heat and simmer 15 minutes. Add pepper.
place 8 ovenproof serving bowls on a baking sheet. Place 1 bread slice in each bowl, ladle soup over bread. Top with 1 cheese slice; sprinkle with parmesan. Broil 8 inches from heat until cheese melts.