Place prawns in a bowl with garlic, ginger, spices, 2 teaspoons oil and some salt and pepper. Stir well to coat, then cover and marinate in the refrigerator for 15 minutes.
Meanwhile, cook the rice according to packet instructions. Cover and keep warm.
Heat 1 tbs oil in a wok or frypan over medium-high heat. Add prawns and stir-fry for 2-3 minutes until lightly golden. Remove with a slotted spoon and set aside. Add the remaining 1 tbs oil to the pan, reduce the heat to medium, then add onion and cook, stirring, for 5 minutes until softened. Stir in the tomato, then cover and simmer over medium heat for 5 minutes until tomato is softened.
Return prawns to pan with yoghurt and cilantro, then warm through, stirring, for 1 minute.