Put the spring onions and milk in a small pan and heat to boiling. Take off the heat and leave to infuse.
Boil whole potatoes in their skins for 30-40 mins, or until tender (timing depends on their size). Drain and peel off the skins. Put the potatoes back in the pan, cover and gently heat for a min or two.
Remove from the heat and mash the potatoes with the butter until no lumps are left. Reheat the milk and the spring onions, then gradually beat this into the potatoes, mixing well with a wooden spoon to make the potatoes fluffy. Season well.
Serve on individual plates with a hollow in the centre of each serving in which you can put a big knob of butter. Diners then dip each forkful into the pool of melted butter before eating.