Grind coconut and kus kus to a fine paste. Chop onion and garlic into very fine pieces. Cut tomatoes. Cut coriander leaves finely. Heat oil in a large kadai. Add chopped onions, garlic and curry leaves till it turns brown. Add turmeric powder and chilli powder. Stir and then add tomatoes. Cook for a few minutes.
Add the coconut paste and prawns. Add required salt and water. Once the prawns are cooked, in a separate kadai roast the mustard and jeera till it splutters. Make a powder and before removing the cooked prawns from the flame, add this roasted powder and mix well. Add lemon juice and stir well all the ingredients. Garnish with chopped coriander leaves.