Heat oil in a pan on medium heat. Add finely chopped onion and fry it until it turns golden brown in colour.
Add chopped ginger and green chilli to it. Fry it for 2 - 3 minutes until it is crisp.
Add thinly sliced apples and salt to it. Gently stir it for 3 - 4 minutes until it is tender.
Switch off the heat and set it aside to cool for half an hour. Once it reaches normal temperature, add beaten curd and mix it well until all the ingredients are well combined (creamy and smooth in consistency).
For the seasoning:
Heat oil in a pan. Add mustard seeds to it. When they crackle, add curry leaves and fry it until it is crisp.
Pour this seasoning over the pachadi. Serve it with rice.