Cook the noodles following pack instructions. Meanwhile heat the oil in a wok and cook the onion for 5 minutes, until beginning to soften. Add the chilli, garlic and lemongrass paste, and cook for a further minute.
Add the beef to the wok and cook for 5 minutes, until browned. Transfer the contents of the wok to a bowl, set aside and keep warm.
Add the peppers, carrot and broccoli to the wok, and fry for 5 minutes, until slightly tender. Add the pak choi and cook for a further 2 minutes, until wilted.
Return the beef to the wok along with the egg noodles and soy sauce, and heat through. Spoon into 4 bowls and serve at once.