Wash and lightly scrape (not peel) the exterior of the hog plums. Score them vertically (apply 3-4 vertical cuts).
Place them in a pan with 2 cups of water and salt and bring the mixture to a boil. Cook until tender - this could take anywhere between 5 mins (if you are using the large variety which are more tender and fleshy) to 15 mins (if you are using the small and hard, non fleshy variety).
Next, add the powdered jaggery and simmer for 7-8 minutes. Do a taste check and add some more. Don't go overboard with the jaggery at this stage. You can always add more when you are actually making the curry.
Turn off the heat and let the hog plums cool down and absorb the flavours for at least 3-4 hours. For best results, once cooled transfer to the fridge and keep overnight.
To make the masala, heat a skillet/tawa on medium and dry roast each of these ingredients one by one - red chillies, coriander, cumin, peppercorns, mustard, garlic, half of the coconut and half of the onion. Let the roasted ingredients cool completely before grinding along with the turmeric and the remaining coconut and onion (the unroasted ones). Use water 1-2 tablespoons at a time to achieve a fine paste. Remove the masala and reserve the masala water from the mixie.
Bring the hog plums once again to a boil, reduce to simmer and add the ground masala paste to it - be careful to keep the heat on very low or else the coconut in the masala could curdle. Keep a spoon or ladle in the pan to help prevent this.
When the curry is simmering, do a final taste check - adjust the salt and jaggery if required. The more you simmer, the more accurate your taste check will be as the flavours that were absorbed by the hogplums will get released into the curry. If the curry begins to taste very tangy try balancing it with some salt or dilute the curry slightly with 2-3 tablespoons of water at a time. Once done, remove from heat
To temper, heat ghee in a small pan and toss in the mustard seeds. When they stop spluttering, pour the mixture into the prepared curry and immediately cover the pan to trap the aroma and flavours.
Serve hot with piping hot rice.