Pre-heat oven to 190C, 190ºC , Gas mark 5. Drain the can of salmon and break the fish into large chunks. Set aside. Mix together the two types of potato. Put the butter or oil into a baking dish and heat in the oven until hot. Pile the potato mixture into the dish, pressing it into a round cake with the back of a spoon and dribbling a little of the hot fat over the top of the cake. Return to the oven and bake for 45 minutes. Arrange the salmon over the cooked potato cake. Sprinkle the cheese over this and return to the oven for a further 15 minutes. Serve with peas, sweetcorn or tomatoes. Serves 4. Approx. 420 kcals per serving