Cullen skink has its roots in Cullen, a small town on the Moray Firth Coast and is traditionally made using Finnan haddock on the bone but a boneless smoked haddock or other smoked white fish can be used but do avoid any fish that has been chemically smoked and then dyed a lurid yellow colour.
Put the potatoes and onion in a large pot with the water and cook until the potatoes are almost soft. Lay the Finnan haddock on top, put the lid on the pot and cook for a further 5 minutes until the fish is just cooked. Remove the fish, skin, bone and flake the flesh.
Partially mash the potatoes and onions in the cooking liquor (some prefer a "lumpy" soup while others like a smoother base). Return the flaked fish to the pot, mix thoroughly and add enough milk to create the desired consistency, either a thickish stew- soup or a thinner soup. Season the Cullen Skink and serve with bread.