Put the sprouts in a large pan, cover with boiling water and cook for 8-10 mins or until very tender. Drain, and leave to steam-dry for a few mins. Tip back into a saucepan and mash to a chunky texture with a potato masher. Add the butter, cream and nutmeg, and season well. Cook for a further 2 mins. Mix in half the chestnuts, then sprinkle over the remaining chestnuts to serve.