Boil the potatoes in salted water for 15-20 minutes until very tender.
Meanwhile, melt half the butter in a separate pan. Add the onion and cook for 5 minutes until soft.
Add the shredded cabbage and cook for 5-8 minutes until wilted and tender. Season well.
Drain the potatoes and mash thoroughly with the remaining butter and the milk until smooth.
Fold the cabbage and onion mixture into the mashed potato along with most of the spring onions.
Serve in bowls topped with an extra knob of butter and the remaining spring onions.