01 - Place the potatoes in a large saucepan, cover with cold salted water, and bring to a boil. Reduce heat and simmer for 15–20 minutes, or until potatoes are fork-tender.
02 - Bring a second pot of salted water to a boil. Add the shredded cabbage and cook for 4–5 minutes until just tender. Drain well.
03 - In a small saucepan, gently heat the milk and butter together until the butter melts. Do not boil.
04 - Drain the potatoes thoroughly and return them to the pot. Mash until smooth and fluffy.
05 - Gradually stir in the warm milk and butter mixture, then fold in the drained cabbage and sliced scallions. Taste and season with salt and pepper.
06 - Serve immediately, with an extra knob of butter melting on top if desired.