Place coriander stalks in a food processor, reserving leaves, with the remaining curry paste ingredients and whiz until finely chopped.
Cook the sweet potato, green beans and corn in slightly salted water for 5 mins, then drain.
Fry the garlic for 30 secs, then tip in the chicken and spring onion. Stir-fry for 2-3 mins. Add curry paste and cook for a min.
Gradually add the coconut milk, stock, lemon grass, lime leaves, sweet potato, corn and beans. Bring to a simmer and cook gently for 6 mins. Add the coriander leaves.