In a heavy frypan saute the onion and the capsicum in 1/2 cup of butter. Cook until tender.
Mix in 1/2 cup of the flour and cook over medium heat for 10 minutes, stirring frequently. Stir in crabmeat then remove from heat and set aside.
In a large saucepan bring the water to a boil. Add the prawns, scallops and flounder then simmer for 3 minutes. Drain, reserving 1 cup of the cooking liquid and set the seafood aside.
In a heavy saucepan melt the remaining 125g butter over low heat. Stir in 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 then cook stirring constantly until the mixture is thickened and bubbly.
Mix in the grated Cheddar cheese, vinegar, Worcestershire sauce, salt, pepper and chilli sauce. Stir in cooked seafood.
Preheat oven to 180 degrees C. Lightly grease one 20x30cm baking dish.
Press crabmeat mixture into the bottom of the prepared baking dish. 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.