Bring a large pan of salted water to the boil, add the sprouts and cook for 3-4 mins until slightly tender. Drain and rinse under cold water.
Meanwhile, very gently fry the onions in the oil and half the butter for about 10 mins until soft. Add the garlic and stock cube and cook for 2 mins, stirring, until everything is combined. Remove from the heat.
Roughly chop the sprouts by hand or in a food processor, then stir them in with the onions and the rest of the butter, along with the double cream and nutmeg. Put back on the heat and cook until the cream is bubbling. Season to taste. Toss in the chestnuts and sprinkle over a little extra nutmeg to serve.