Place the noodles in a heatproof bowl and cover with boiling water. Stand for 5 minutes or until soft. Drain.
Heat half the oil in a wok over high heat. Stir-fry the chicken in two batches until browned, about 3-4 minutes. Transfer to a plate.
Heat the remaining oil in the same wok. Stir-fry the onion and carrot for 3-4 minutes. Add the garlic and broccoli and stir-fry for 2 minutes.
Add the combined sauce and lemon juice and stir-fry for 2-3 minutes or until vegetables are just tender. Add the choy sum, cooked chicken and drained noodles and toss until chicken is cooked through and choy sum is wilted. Serve.