Cook the potatoes in a pan of boiling water until tender, about 10-15 mins. Meanwhile, chop the spring onions, separating the green part from the white.
Lay the cod on a piece of kitchen paper to remove any excess moisture. Heat a frying pan to a high heat, add the oil, then the fish, skin side down. Don't move the pan or even shake it as this will tear the skin. Cook for 4-5 mins until the edges start to colour. Turn the fish over and cook for a further 3-4 mins.
Bring the cream and milk to the boil in a pan with the butter, then add the chopped white part of the onions, simmer for a few minutes, then season, if you want to. Mash the potatoes and mix in the hot cream and milk and half the chopped green part of the spring onions. Sprinkle the rest over the mash. Serve with the fish, pouring over any pan juices.