1.Heat oil in a large wok on medium. Saute garlic 1 minute, until fragrant.
2.Increase heat to high. Add lamb and stir-fry 2-3 minutes, until browned all over. Add broccoli and mushrooms. Stir-fry further 2-3 minutes.
3.To make sauce, dissolve cornflour in vinegar. Add remaining ingredients. Pour sauce into wok and bring to simmer. Simmer 2-3 minutes, stirring occasionally, until sauce thickens.
4.Sprinkle lamb with green onion and serve with steamed rice.
Lamb steak can be replaced with a lean, tender cut of beef such as fillet, sirloin or rump.