Mix the meat with the cornflour, sherry, water, soy sauce and sugar in a small bowl; set aside.
In a large frying pan or wok, heat the oil over medium-high heat. Cook the mushrooms until browned and tender, about 7 minutes. Remove from the pan, cover and keep warm.
Add the beef and sauce to pan; cook until browned on all sides, 3 to 5 minutes. Return the mushrooms to the pan and add the beansprouts; heat through.
Tip
Instead of mushrooms, use a packet of your favourite stir fry veg.