Combine lamb, ketcup manis and sweet chilli sauce in a medium bowl, mix well to combine. Cover and place in the fridge for 10 minutes.
Heat a wok over high heat until hot. Add sesame seeds and stir-fry 2 minutes or until golden. Remove to a plate.
Add 3 teaspoons oil and half the lamb and stir-fry 2 minutes or until brown. Remove to a plate and repeat with oil and remaining lamb.
Add remaining 2 teaspoons oil to the wok and heat until hot. Add onion and stir-fry for 1 minute. Add the mushrooms and stir-fry 2 minutes. Add bok choy stems, snow peas and stir-fry 1 minute.
Add lamb and bok choy leaves and stir-fry 1-2 minutes or until heated through. Stir through the sesame seeds and serve immediately with rice.