Scrub the potatoes and cook them in their jackets. Put them in a saucepan of cold water, add a good pinch of salt and bring to the boil. When the potatoes are about half cooked, 15 minutes approx. for ‘old’ potatoes, strain off two-thirds of the water, replace the lid on the saucepan, put onto a gentle heat and allow the potatoes to steam until they are cooked.
Remove the dark outer leaves from the cabbage. Wash the rest and cut into quarters, remove the core and cut finely across the grain. Boil in a little boiling salted water or bacon-cooking water until soft. Drain, season with salt, freshly ground pepper and a little butter. When the potatoes are just cooked, put on the milk and bring to the boil. Pull the peel off the potatoes, mash quickly while they are still warm and beat in enough boiling milk to make a fluffy purée. (If you have a large quantity, put the potatoes in the bowl of a food mixer and beat with the spade.) Then stir in the cooked cabbage and taste for seasoning. Cover with tin foil while reheating so it doesn’t get crusty on top.
Colcannon may be prepared ahead up to this point and reheated later in a moderate oven, 180°C/350°F/regulo 4, for 20–25 minutes approx.
Serve in a hot dish or with a lump of butter melting in the centre.