Cut each of the pork fillets across into 5mm slices, then cut each slice into 5mm strips. Cover and set aside.
Cook the noodles in a saucepan of boiling water for 4 minutes, or cook or soak them according to the packet instructions. Drain well and set aside.
While the noodles are cooking, heat a wok or a large heavy-based frying pan until hot. Add the oil and swirl to coat the wok, then add the onion and garlic and stir-fry for 1 minute. Add the five-spice powder and stir-fry for another minute.
Add the pork strips to the wok and stir-fry for 3 minutes. Add the snow peas or sugar snap peas and the capsicums and stir-fry for a further 2 minutes. Pour in the stock, stir well and bring to the boil.
Add the noodles to the wok and stir and toss for 2–3 minutes or until all the ingredients are well combined. Season to taste and serve immediately, sprinkled with coriander leaves.