Sauté onion in butter in a large Dutch oven until tender. Dissolve bouillon cube in hot water; gradually add bouillon and half-and-half to sautéed onion, stirring well. Stir in parsley, salt, celery salt, mace, pepper, and crab meat; simmer over low heat 15 minutes, stirring constantly.
Combine flour and 2 tablespoons water, stirring to make a smooth paste. Stir paste into soup, and cook until slightly thickened. Remove from heat; stir in sherry. Ladle into individual soup bowls, and serve immediately.