Put the potatoes in a large pot with cold water and a pinch of salt. Bring to the boil and simmer for 15-20 minutes until tender. Drain well and let steam dry.
While the potatoes cook, melt 25g butter in a large pan over medium heat. Add the shredded cabbage and cook for 5-6 minutes until softened but still with some bite.
Add the spring onions to the cabbage and cook for another minute. Set aside.
Return the potatoes to the pot. Add 75g butter and mash until smooth.
Gradually add the warmed milk while mashing until you reach your preferred consistency.
Fold in the buttered cabbage and spring onions. Season generously with salt and pepper.
Serve in a bowl with a knob of butter melting in the centre.
Notes
For ultra-smooth mash, use a food mill or ricer instead of a masher
Don't skip warming the milk – cold milk can make potatoes gluey
Leftover colcannon keeps for 3 days in the fridge in an airtight container