Combine the marinade ingredients in a bowl. Add in the lamb strips and leave to marinate for 10 minutes while you prepare the vegetables and noodles.
Thinly slice the red peppers, leeks and onion and set aside. Cook the noodles as per packet instructions. When they are cooked add in the scallions, coriander and soy sauce and stir well to combine.
Heat a little oil in a wok or frying pan and stir-fry the peppers, leeks and onion for a couple of minutes. Remove to a plate and keep warm while you cook the lamb. Add a little more oil to pan and when it is very hot stir-fry the lamb in batches. As it cooks remove it to a plate and keep warm.
Return the vegetables and lamb to the wok and mix well together. Remove to a serving dish and sprinkle over the sesame seeds. Serve immediately with the noodles.
To toast sesame seeds: Heat a small dry pan over low heat. Add sesame seeds and stir constantly until golden. This will only take about 2 minutes. Transfer to a small bowl and let cool.