Wash the prawns, pat them dry and sprinkle turmeric powder and salt to taste. Keep aside. Skip this step, if you are not using prawns.
Wash the potatoes thoroughly under running water before peeling the skin. Once peeled, keep the skin soaked in water for 10 minutes. After 10 minutes, discard the water and again rinse the peels.
Put the potato skins in a steamer or put them in a tiffin box, transfer to a pressure cooker and cook for ten minutes. Once the steam has released and the potato skin has softened, let it cool down.
In a grinder, add the potato skins, coriander leaves, green chillies, garlic and a pinch of salt. Grind to a coarse paste.
In a frying pan or kadai, heat 1 tbsp of mustard oil and when the oil has reached a smoking point, add the prawns and lightly saute them and keep aside. Skip this step and directly go to the next step if you are not using prawns.
In the same oil, add another 1/2 tsp of mustard oil. When it's hot, add nigella seeds and more green chillies if you wish to.
Add the potato skin paste to the oil and cook on high flame. Stir continously, till the water has evaporated. Add the prawns and add sugar.
Keep stirring or else it sticks to the bottom. Once it as become mushy, add salt to taste and drizzle half tsp of mustard oil from top.
Turn off the heat and serve this with steamed white rice.