Heat a wok over a high heat. Add the groundnut oil and, when it is very hot and slightly smoking, add the ginger, garlic and spring onions. Stir-fry for 20 secs, then add the prawns. Stir-fry for about 1 min, then add all the sauce ingredients, along with ½ tsp salt and ½ tsp ground black pepper. Continue to stir-fry for another 3 mins over a high heat. Serve at once, scattered with the coriander and spring onion.