Peel and finely slice the onions. Warm a deep casserole dish or pan over a medium-low heat for 2 mins. Add the butter, onions and a pinch of salt and pepper. Pop on a lid, gently cook, stirring every so often, for 15-20 mins till the onions have softened and browned a little. While the onions cook, trim the dry ends off the cavolo nero and finely shred the leaves.

When the onions are soft, add the cavolo nero with 100ml cold water. Cover and sweat for 5-8 mins, stirring once or twice, till the cavolo nero is bright green and wilted. Turn the heat down to low.

Stir the mustard into the pan. Pour in 1½ pots of cream. Gently cook and stir for 2-3 mins to warm the cream (don’t boil it – just slowly warm till it’s steaming hot). Take off the heat and finely grate over half the nutmeg. Taste the cavolo nero and add a little more nutmeg, salt or pepper if you think it needs it. Spoon into a warm serving dish and serve.