Place grated coconut, ginger, garlic and a little water in the bowl of a food processor. Process until smooth. Set aside.

Heat oil in a large pan. Splutter mustard seeds and fenugreek seeds. Add onion, and sauté, stirring occasionally, for 2-3 minutes or until browned. Add curry leaves. Reduce heat to low. Add ground turmeric, ground coriander, and chilly powder. Fry for a minute. Add tomatoes, and saute until it starts to breaks down.

Tip in the ground coconut mixture. Fry for a further 2 minutes. Pour enough water to make a thick sauce. Increase heat to medium-high, and bring it to a boil. Add enough salt. Simmer for 2-3 minutes. When oil separates, mix in coccum. Mix in the prawns and few more curry leaves. Bring to a boil again, then reduce heat to a simmer. Cook, covered, for 4-5 minutes or until prawns are just opaque and cooked through. Serve hot rice or roti.