Melt butter in saucepan over medium heat. Add onions; cook slowly until tender (about 25 minutes.)
Blend in flour and black pepper. Stir in broth and water. Bring to a boil and simmer for 30 minutes.
Add sherry and ladle into ovenproof bowls.
Float bread cubes and sprinkle with cheeses.
Broil until cheeses melt and begin to brown.
