Fish cottage pie

Season the fish lightly with salt and pepper and cook gently in the milk until done. Drain, keeping the milk in another container. Add extra milk to make up 500 ml. Flake the fish and set aside. Make a white sauce with the margarine, cake flour and milk, and season with mustard powder, mayonnaise, salt and pepper. Add the flaked fish. Cheddar cheese, chives, pepper and mushrooms and mix well. Spoon into a greased ovenproof dish. Mash the potatoes and add the margarine and milk. Season with salt and pepper. Spread the mashed potato on top of the fish layer and pipe potato rosettes on top. Garnish, if desired, with red pepper and mushrooms. Place the dish under the grill until the potatoes just begin to turn golden brown and dish is heated through.
Serves 6.