Start by making the Roti. Mix the rice flour with the coconut and salt. Add the water and mix, then use your hands to squish the dough together and make a ball. Separate the ball into 4 separate ones and set aside.
In medium sized frying pan toast both the mustard and cumin seeds along with the cardamon pods and curry leaves until fragrant. Remove form the pan and set aside.
Next, cut the woody ends of the lemongrass stick off and remove the outer leaves. Place it in a food processor along with all the other paste ingredients and blend until everything is combined.
Heat a little oil in the pan and add the onion. Cook on a low heat for two minutes then add the tomatoes cook for a further two minutes. Add the paste and cook for one more minute, stirring as you go.
Pour in the coconut cream and bring to a gentle boil. Cut the fish into 1 inch square chunks and add those too. Let the fish cook for 5-6 minutes until cooked through.
Meanwhile heat a large frying pan on a high heat with a little oil. On a sheet of baking paper take two of the roti balls and squash them down with the palm of your hand until they're about 1/4 inch thick. Carefully remove them from the paper with a spatula and transfer them to the pan. Cook for a few minutes on each side until slightly browned. Do the same with the rest of the dough adding more oil as necessary.
Spoon the curry into bowls topping them with the prawns and lime. Serve with or without rice.