1. Remove the shrimp from their shells, leaving the tails intact. Devein and then slit them down the underside all the way from head to tail.

2. Heat the oil in a wok or heavy skillet. Add the onion and fry until soft and golden. Add the chopped lemon grass, sliced red chilies, ginger, cumin, and cilantro, and sauté for 2 minutes.

3. Add the fish sauce and coconut milk to the wok. Stir well and then add the peanuts and chopped tomatoes. Cook gently over low heat until the tomato is soft and the flavors of the sauce are well developed.

4. Stir in the prepared shrimp and simmer gently for 5 minutes, or until the shrimp are pink and tender. Add the sugar and transfer to a serving dish. Serve hot sprinkled with lime juice and chopped cilantro leaves.