Pre -heat oven to 200C (400F/Gas 6)
With the pointer of a sharp knife, cut small incisions in the beef and insert thin slices of garlic. In a bowl, mix together five-spice powder, 1 teaspoon salt and pepper, then rub into beef. Place in a baking dish and cook for 45 minutes, turning beef and basting at least once.
Boil potatoes in salted water for 10 minutes. Drain well: when cool enough to handle, slice. Add to baking dish with mushrooms and cook for a further 15 mintes.