Melt the butter in a large soup pot over medium heat. Add the onions and cook until soft and fragrant, about 5 minutes. Add the carrots, celery, and potato. Cook 5 minutes. Stir in the flour and Old Bay seasoning. Cook 1 minute. Whisk in the sherry.

Add the tomatoes, water, and liquid from the crab and clams. Bring to a boil, then reduce heat an simmer for 15 minutes or until vegetables are soft. Add the seafood; simmer 10 minutes. Stir in the parsley.

Cut the center out of each roll. Scoop out some of the bread to make room for the soup, being careful to leave at least a half inch of bread around the bottom and sides to act as a barrier for the soup. Ladle soup into bread bowls. Serve immediately.