Peel the tiger prawns, cutting down through their back and removing the intestinal tracts and reserving the shells and heads. Put the peeled prawns in a bowl of iced salted water and rub them gently between your fingers. Drain.
In a saucepan, heat the oil and fry the onion until translucent. Add the lemon grass, garlic, green chillies and the prawn shells. Stir until the shells blush pi