From 1922:

Take ½ lb. cooked fish, ½ lb. potatoes, 1 dessertspoonful butter, 1 teaspoonful flour, 1 gill fresh milk. Have the fish from the skin and bones. Boil and mash the potatoes with a dessertspoonful of milk; put the butter and flour in a saucepan, and mix on the fire till smooth, then stir in the milk; boil and season with salt and pepper. Put a layer of half the fish in the bottom of a piedish, pour over half of the sauce, then the rest of the fish, and cover again with sauce. Spread the potatoes on the top, with little pieces of butter, mark and put in the oven for about half an hour.