Place the potatoes in a large pan of salted water. Bring to the boil, cover and simmer for 15-20 minutes or until the potatoes are tender without breaking up. Drain and return to the pan over a low heat to dry out.
Just before the potatoes are ready, place the milk and spring onions or scallions in a small pan and simmer gently until the spring onions have softened in the milk, then remove from the heat.
Mash the potatoes or pass them through a potato ricer if you want a really smooth result. Using a wooden spoon, beat in the butter until melted and then beat the milk and spring onion mixture until you have achieved smooth, creamy mash. Season to taste.
To serve, transfer to a warmed dish and add a nob of butter or use as required.