Place the green mango / kairi in a heatproof glass dish. Add turmeric powder, cumin seeds, mustard seeds / kalonji, and coriander seeds. Mix well and set aside.
Heat the mustard oil in a frying pan or wok / karahi, and add the garlic cloves followed by the black mustard seeds / rai, curry leaves, fenugreek seeds, and whole red chilies. Fry for about 1 – 2 minutes till the garlic cloves are light golden brown, and the curry leaves start to splutter.
Carefully pour the hot oil over the green mango / kairi mixture. Let it sit for 2 minutes, and then mix through.
Once the achar has cooled down, add vinegar and salt to taste.
Store the rishte ka achar / grated mango pickle in a glass airtight container in the fridge. As long as it's completely covered with oil, it can keep in the fridge for a couple of weeks. Make sure to use a clean and dry spoon to take out the achar.
Note: It’s important to take a heatproof glass dish to make this achar, and also make sure that the glass dish is small and deep so that the mustard oil covers the green mango / kairi completely. If the green mango / kairi isn’t covered by the oil, it can develop mold.