Place the beef short ribs in a large bowl.
Peel and chop the garlic and slice the ginger into discs.
Mix the garlic, ginger, five-spice, sherry, soy sauce, sugar, and 20ml of water.
Pour the marinade over the beef and marinate for at least 1 hour or overnight.
Preheat the oven to 150°C.
Transfer the ribs and marinade to an ovenproof dish and simmer on the hob for 5 minutes.
Cover tightly with foil and bake for 3 hours until the meat is tender.
Remove the ribs from the cooking juices and keep them warm in the turned-off oven.
Sieve the juices into a pan and stir in a paste made from cornflour and 2 tablespoons of water.
Add halved mushrooms and simmer for 5 minutes until the sauce thickens.
Season to taste and pour the sauce over the beef to serve.