Combine the kecap manis, lemon juice and fish sauce in a small jug. Place the noodles in a large heatproof bowl and pour over enough boiling water to cover. Set aside for 8-10 mins or until tender. Drain well.
Meanwhile, heat half of the oil in a large frying pan over high heat. Add carrot and stir-fry for 2 mins or until almost tender. Add choy sum and stir-fry for 1 min. Transfer to a plate.
Heat remaining oil in the wok or pan. Add chicken and spring onion and stir-fry for 5 mins or until the chicken is charred and heated through. Add the noodles and egg and stir-fry for 30 secs or until the egg is just set. Return the choy sum mixture to the pan with the kecap manis mixture. Stir-fry for 30 secs or until combined.
Divide the noodle mixture and bean sprouts among serving bowls.
Serve with lemon wedges and coriander sprigs