01 - Place the potatoes in a large pot, cover with cold salted water, and bring to a boil. Reduce heat and simmer for 15–18 minutes, or until fork-tender.
02 - Meanwhile, in a separate pot, melt half the butter over medium heat. Add the cabbage and cook, stirring, for 5–7 minutes, until soft but not browned. Stir in the spring onions and cook for 1 minute more. Remove from heat.
03 - Drain the potatoes well and return to the pot. Mash thoroughly until smooth.
04 - Gently heat the milk, then add it to the potatoes along with the remaining butter. Mix until creamy.
05 - Fold in the cooked cabbage and spring onions. Season generously with salt and pepper.
06 - Serve in a warm bowl with extra butter melting on top.