1. Mix together the Thai paste and lime juice in a bowl. Add the prawns and marinate for 25 minutes (setting aside 1 tbsp marinade to dress the dish later).
2. Heat 1 tbsp oil in a wok and stir-fry the peppers and pak choi for 3-4 minutes until tender. Remove and set aside. Add the noodles to the pan and stir-fry for 4-5 minutes. Stir in the peppers and set aside.
3. Mix the cucumber and pickled ginger in a small bowl.
4. Heat the remaining oil in the wok and stir-fry the prawns for 3-4 minutes until they are pinkish and cooked through.
5. Place one half of the pak choi on each plate and top with the noodles and peppers. Add the prawns and use the marinade set aside earlier to dress the dish. Scatter over the pickled ginger and cucumber, and sprinkle with toasted sesame seeds.