Strain oyster liquid into a measuring cup. Chop oysters coarsely. Heat liquid over medium heat, add chopped oysters and simmer for 5 minutes. Remove oysters and reserve. Add hot water to the reserved liquid to make 5 cups.
Melt butter in a large saucepan over medium heat. Add flour gradually, stirring constantly until smooth. Gradually add the hot liquid, whisking constantly, and cook until smooth. Add onion, parsley, salt and pepper. Simmer for 15 minutes. Add reserved oysters and heat thoroughly. Serve immediately.
Contributed by Joyce Taylor