Place the onion, garlic, lemon juice, ginger, turmeric, chilli powder and curry paste in a food processor and blend until fairly smooth. Season with salt.
Heat the oil in a wide saucepan over a medium heat. Add the onion paste and stir-fry for 2–3 minutes. Add the prawns and stir-fry for a further 4–5 minutes until they turn pink and are cooked through.
Remove from the heat and stir in the coriander and spring onions. Serve immediately.