Place a large wok or large skillet over high heat and add half the oil to pan. Add beef strips and stir-fry for 1 minute, until strips are golden on the outside and still pink in the centre. Transfer to a heat-proof bowl.
Add remaining oil to the pan, along with the carrot, capsicum and broccolini. Stir-fry for 3 - 4 minutes. Add onion and cook a further 2 - 3 minutes, until veggies are tender-crisp.
Return beef to the pan, add oyster sauce and kecap manus, and toss to coat. Serve stir fry over steamed rice, with spring onions and lemon wedges.
Switch rump steak for scotch fillet or stir fry strips.