Grind together small onion,ginger, garlic, chilli, pepper and turmeric powder, mustard and fenugreek seeds with 1-2 tbsp water to a fine paste. Heat oil in a pan & saute the onion. Add the ground masala. Once oil starts appearing add chopped tomatoes, curry leaves and salt. After 5 minutes add prawns & coconut bits, mix well. Add water & cook.Cook till the prawns are done and covered in masala. Remove from fire.Serve hot with rice or roti. Garnish with curry leaves.