Scrub the potatoes clean, then put them into a large saucepan, with the larger spuds at the bottom. Cover with cold, salted water and bring to a boil, then simmer for about 15 minutes, until just soft but not cooked through.
Tip out two-thirds of the water, cover and simmer until the potatoes are cooked through – keep an eye on the pan to make sure it doesn’t boil dry.
Blanch the greens in salted, boiling water, until tender but not overly soft. Drain and finely chop with the spring onions (or blitz briefly in a food processor until chopped, but not pureed).
Once the potatoes are done, drain and put them back in the hot pan. Cover with a tea towel and leave to steam for five minutes.
In the meantime, melt the butter and bring the milk or cream to a simmer; keep both warm. Put a serving dish into a warm oven or fill with boiling water.
Peel the potatoes, holding them in a tea towel to protect your hands, then mash or pass through a ricer. Beat in the melted butter, followed by the milk, until the mash is of a consistency you like.
Stir in the greens and season to taste. Spoon into the warmed dish, top with a big knob of butter and serve immediately.