Pour boiling water over the rice noodles in a large heatproof bowl and leave for 3-5 mins to soften. Meanwhile, in a large wok or frying pan, heat the oil, add the leek and stir-fry over a high heat until starting to soften. Add the green beans and beansprouts, then cook for another 3 mins. Add the garlic and prawns, and cook until the prawns are starting to turn pink. Splash in the soy sauce and heat until everything is cooked through.
Tip the contents of the wok onto a large serving dish, drain the rice noodles, add to the serving dish, then toss everything together. Top with the toasted sesame seeds and pickled ginger to serve.