Chop all ingredients. Saute mushrooms and onions 5 minutes in butter. Stir in bread crumbs and crabmeat.Cook 2 minutes. Add salt, pepper, lemon juice, and ketchup. Pour in cream 1/2 and 1/2. Put in flounder. Bake 425 for 10-15 minutes/ Pat of butter on top and serve with mornaise sauce.