Free the fish from all skin and bone and break it into little flakes. Shell the prawn, remove the heads and tails and boil them, then chop them.
Mix the fish and the prawns together; add onions, sauce and mustard and season with the powdered ingredients and salt. Stir in the eggs well beaten and if mixture is not sufficiently moist, add fish stock or milk.
Pour the mixture into greased pie-dish, scatter over with breadcrumbs and put a few pieces of dripping on the top.
Bake until nicely brown. Serve either hot or cold.