Sauté onion, shallots, and garlic in butter in a large skillet until tender; add oysters, shrimp, lobster, and crayfish. Cook over medium heat 5 minutes, stirring frequently. Stir in flour, and continue cooking 5 minutes. Add fish stock, tomatoes, salt, and pepper; cook over medium heat 20 minutes, stirring frequently. Stir in saffron; simmer 5 minutes.
Place redfish in a 13- x 9- x 2- inch baking dish. Bake at 350° for 20 minutes or until fish flakes easily when tested with a fork. Place each fillet in an individual serving bowl; pour hot seafood mixture over each fillet.