Put onions, leeks and oil in a large, heavy saucepan. Cook slowly until onions are tender but not brown. Add wine, salt, pepper, garlic and tomatoes. Cook over low heat for 10 minutes. Heat butter in a skillet; add flounder, clams in shells, cod and lobster. Cover and cook slowly for 15 minutes. Blend flour and water. Stir into the tomato sauce. Add fish; heat. Serve in bowls sprinkled with parsley.