1. Remove any pin bones and cut the fish fillets into large cubes.
2 Cut the potatoes into even sized pieces and boil for about 10 -15 minutes.
3. Meanwhile, put the butter or margarine, flour and milk in a large saucepan.
4. Cook over a low heat, stirring all the time, until the sauce has thickened.
5. Stir in the fish, peas and seasoning and continue cooking over very low heat for 5-8 minutes or until the fish is cooked, stirring occasionally.
6. Drain and mash the potatoes. Pour the fish mixture into a shallow oven-proof dish and spoon or pipe the mashed potatoes on top.
7. Place on a baking tray and cook under a low to medium grill until the topping is golden brown. Serve with a selection of vegetables.