Boil a pan of water, add the sprouts and blanch for 6 mins. Drain and cool quickly under the cold tap or in iced water. To get ahead, pack into a plastic food bag and chill overnight.
On the day, heat the goose fat in a large wok and add the sage and chestnuts. Cook for a few mins over the heat, add the sprouts, season and stir-fry until piping hot.