Heat a wok or frying pan over a high heat. When hot, add 2 tsp oil and swirl around wok. Stir fry half the beef for 2-3 minutes or until browned. Transfer to a plate. Repeat with remaining oil and beef.
Add onion to wok and stir fry for 2 minutes or until softened. Add capsicum and snow peas. Stir fry for 2 minutes or until just tender. Return beef to wok with char siu sauce and vinegar. Toss until well combined. Serve stir fry with rice and sprinkle with cashews.
Ask your butcher to dice the rump steaks into cubes to save time.