Place the washed basmati in a heavy-based medium saucepan. Add the juice of 3 limes, the chopped coriander stalks, 400ml coconut milk and salt to taste. Add just enough water to make the liquid in the pan come level with the rice.
Bring the rice to the boil and simmer for 5 minutes. Cover tightly, remove from direct heat and set aside for 15 minutes by which time the liquid will be absorbed and the rice cooked through.
Toss the prawns with the remaining lime juice and set aside to marinate briefly.
Heat the oil in a heavy-based saucepan. Add the lemon grass, ginger, garlic, shallot, chillies, garam masala and turmeric and fry, stirring often, until the shallot has softened.
Add the remaining coconut milk and the plum tomatoes. Season with salt and freshly ground pepper. Simmer for 5 minutes.
Increase the heat and add the king prawns. Cook for a further 2 minutes, turning the prawns to ensure even cooking.
Serve the prawns with the coriander rice, garnished with the coriander leaves.