Lightly grease a 13x9x2-inch baking dish; set aside.
In a large saucepan, sauté the onion and the pepper in 1/2 cup of the butter or margarine until tender.
Stir in 1/2 cup of the flour and cook over medium heat for 5 minutes, stirring frequently. Add the crabmeat and mix well. Press this mixture into the bottom of the prepared baking dish and set aside.
In a large Dutch oven, bring the water to a boil. Add the shrimp, scallops and flounder. Simmer for 3 minutes. Drain, reserving 1 cup of the cooking liquid. Set the seafood aside.
Preheat oven to 350° F.
In a large saucepan, melt the remaining 1/2 cup butter over low heat. Stir in the remaining 1/2 cup flour. Cook and stir constantly for 1 minute. Gradually add the milk plus the 1 cup reserved cooking liquid. Raise heat to medium and cook, stirring constantly, until the mixture is thickened and bubbly. Stir in the shredded cheddar cheese, vinegar, Worcestershire sauce, salt, pepper, and hot sauce. Add the cooked seafood and stir gently.
Spoon the seafood mixture over the crabmeat crust and sprinkle with the Parmesan cheese. Bake in the preheated oven for 30 minutes or until lightly browned. Serve immediately.Makes 8