Boil the potatoes in salted water for 15 to 20 minutes until a fork slides in easily. Drain well and leave them in the colander for a minute so the steam escapes.
While the potatoes drain, put the milk, butter and most of the spring onions into a small pan over a low heat until the butter melts and the milk is steaming but not boiling. Take off the heat and leave the onions to soften in the warm milk for 5 minutes.
Return the potatoes to the pan and mash until smooth, then beat in the warm milk a splash at a time until the mash is creamy and almost pourable. It should be softer than normal mash, that is the whole point of champ.
Season generously with salt and pepper, spoon into a warm bowl and scatter over the remaining raw spring onions. Serve as a side, or as a base for sausages, stews or a fried egg.