Put the potatoes in a large pan of salted water, cover with a lid and bring to the boil. Simmer for 15 mins or until cooked through. Drain and leave to steam-dry for 5 mins.
Heat the butter in a large pan, add the spring onions and cook for 3 mins, stirring regularly. Tip in the Brussels sprouts and cook for a further 8 mins until soft. Add the milk and warm through for 1 min more.
Tip the potatoes back into their pan, mash, then add the milk and sprout mixture. Add the nutmeg, season and stir.