Cullen Skink - an odd name for a dish. It comes from the fishing village of Cullen in North Scotland, and 'skink' is thought to derive from the old Scots name for a soup or broth made from shin of beef.
Peel and chop the onion. Melt the butter in a large pan on medium heat. Add the onion and cook until softened but not browned, about 5 minutes. Peel and dice the potatoes into cubes about 1cm / half an inch. Pop them in with the onion, stir and cook for a couple of minutes. Add the stock, the milk, and the bay leaf, then lay the fish on top. The fish will cook in about 5 minutes. You can tell when it's cooked because the flesh will become opaque and will easily break into flakes. Remove the fish and set aside.
Put a lid on the pan and continue simmering until the potatoes are tender, about 15-20 minutes. Separate the fish from its skin, break into flakes, and add to the pan. Taste the soup and add salt if required - it may not need it, because the fish can sometimes be quite salty.
Ladle a pile of potato, onion and fish into a bowl, and then surround it with soup. Sprinkle chopped parsley or chives on top and serve with some warm crunchy bread.