Make VEGEMITE stir fry sauce: Combine all ingredients in a bowl. Mix well.
Heat 1/2 the peanut oil in a wok over high heat. Add 1/2 the beef rump steak. Stir-fry for 2 to 3 minutes or until browned. Transfer to a heatproof bowl. Repeat with remaining oil and beef.
Add onion to wok. Stir fry for 2 to 3 minutes or until onion softens. Add capsicum. Stir fry for 1 minute. Return beef to wok with choy sum, snow peas and red chilli, if using. Stir-fry for 1 minute. Add VEGEMITE stir fry sauce. Stir fry for 1 to 2 minutes or until heated through.
Serve stir fry with rice.