Chop 1 spring onion and tip into a food processor. Slice the remaining 2 spring onions diagonally and set aside. Halve both chillies lengthways and discard the seeds. Put 1 chilli into the food processor, finely chop the other and set aside.
Add the lime zest, sugar, 2 tbsp of the coriander and a generous pinch of sea salt to the spring onion and chilli in the processor and whizz to a coarse paste. Add the prawns and whizz until everything is chopped but still has some texture.
Tip the mixture on to a floured surface and with floured hands lightly shape into 4 rounds, each about 1cm thick. (You can make ahead up to this point. Cover and keep refrigerated for up to 24 hours.)
Heat 2 tbsp of the sunflower oil in a smallish frying pan and start frying the prawn cakes – cook over a medium heat for 3-4 minutes on each side, turning them over when they become golden. As soon as you have turned the prawn cakes, put the noodles into a saucepan of boiling water on a back burner. Stir to separate and cook for 4 minutes.
Meanwhile, heat the remaining 1 tbsp sunflower oil in a wok (or another frying pan) and stir-fry the garlic, ginger and reserved chilli for 1 minute. Tip in the mangetout and reserved spring onions and stir-fry for a further 2 minutes. Remove from the heat and stir in the soy sauce, sesame oil and lime juice.
Drain the noodles and toss them into the stir-fry. Pile on to plates, top with the prawn cakes and sprinkle with toasted sesame seeds and remaining coriander.