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.