Cook the noodles as per packets instructions.
To prepare the lemongrass, cut off the lower bulb and remove any tough, outer leaves. The lower end of the main stalk (the yellow section) is what you use. Chop this finely. Then mix together with a tablespoon of oil, chilli, garlic, ginger and half the lime juice.
Add in the lamb and leave to marinate for as long as you can. Heat a little oil in a wok or large frying pan and stir-fry the onion, broccoli and pepper for a couple of minutes. Remove from the pan and keep warm.
Reheat the pan, add a little more oil and stir-fry the lamb in batches, until browned. Return all the lamb back into the wok and stir through the sweet chilli sauce, fish sauce, lime zest, remaining lime juice, stock or water and season with salt and black pepper.
Then add in the vegetables and noodles and stir-fry until well mixed and hot.
Add the herbs, toss gently and serve immediately sprinkled with peanuts.