Place a frying pan on medium flame. Once hot, add oil and let it warm up slightly. Heating the oil well ensures that the spices release their flavour properly when added.
Add cumin seeds, mustard seeds, dry red chillies, and curry leaves to the hot oil. Let them crackle and release their aroma. This step forms the base flavour of the kadhi, so give it a few seconds before moving on.
Add finely chopped onions, turmeric powder, and salt. Stir well and cook on medium flame for 2-3 minutes until the onions soften. Do not let them brown; the aim is to keep them soft while still adding a gentle bite.
Turn off the flame and allow the onion-spice mix to cool slightly. Adding it directly to the curd while hot may curdle the mixture, so let it rest before combining.
Whisk the curd in a bowl until smooth and lump-free, and then add the cooled onion-spice mixture to the curd. Stir well so the flavours blend evenly. At last, taste and adjust salt if needed, and your quick kadhi is ready to serve.