Marinate the prawns with salt and a pinch of turmeric, keep aside.
Soak the black and yellow mustard seeds in warm water for 15 odd minutes. Drain the water and grind to a smooth paste along with 4-5 green chilies.
In a big bowl, take the mustard paste, 1/2 a cup of coconut paste, turmeric powder and 3 tbsp mustard oil. Sprinkle in the sugar and salt, give it a hearty mix.
Add the prawns to the same bowl, throw in the green chilies. Coat the prawns evenly with the masala.
Heat a kadai, add the prawns along with the masala. Cook over a low flame for 10-15 minutes or till the prawns are just cooked. Remove from heat.
Adjust seasonings. Finish with the remaining grated coconut and mustard oil. Give it a hearty stir, allow to rest for another 5-7 minutes.
Serve hot with rice.