After peeling and removing the red fleshy parts of watermelon, cut the rind into 2" cubes.
Cut a raw mango into cubes.
Mix rind and mango in an earthen pot. Pour 2 cups of water, turmeric and 1/2 tsp of salt and boil until rind is cooked. To check if rind is cooked, insert a knife and it should easily go through it.
Meanwhile grind coconut, raw rice, peppercorns, cumin, green chillies, 3 curry leaves with water to a fine paste.
Add the ground paste to cooked rind and mango and boil for a few minutes. Add water if needed. Adjust salt and take off the flame.
Splutter mustard seeds in coconut oil, add curry leaves and red chillies and pour over the curry.
Substitutions - Watermelon rind can be substituted for ash gourd. Recipe can be scaled up for large quantities.
Raw mango - Make sure to use real sour ones for flavour. If you don't want the curry to be too sour, increase the quantity of rind to 330 gms and add only 20 gms of raw mango.
Ground paste - Coconut and other ingredients should be ground to a very fine paste either in a grinder or on a stove.
Storing - Once the curry cools to room temperature, transfer to airtight container and store in the fridge for upto 3 days. Bring to room temperature, warm it up on low flame and temper before serving.