In a large bowl, whisk together the yogurt, besan, and water until completely smooth with no lumps.
Add the jaggery (or sugar), turmeric, salt, and ginger paste. Whisk again to combine.
Cook the Kadhi
Pour the mixture into a medium saucepan and place over medium-low heat.
Stir continuously in one direction until the kadhi comes to a gentle simmer, about 10-12 minutes.
Reduce heat to low and cook for another 5 minutes, stirring occasionally, until slightly thickened and pale golden. Taste and adjust salt and sweetness.
Make the Tadka
Heat ghee in a small pan over medium heat. Add mustard seeds and wait for them to splutter, about 30 seconds.
Add cumin seeds, curry leaves, dried red chili, and asafoetida. Stir for 15-20 seconds until fragrant.
Pour the tadka immediately over the kadhi and stir to combine. Serve hot.
Notes
Use room-temperature yogurt to prevent curdling. Cold yogurt straight from the fridge increases the risk.Whisk out every lump of besan before heating — lumps cannot be fixed once the kadhi is on the stove.Cook on medium-low heat only. High heat causes the yogurt to curdle and the kadhi to turn grainy.Vegan swap: Replace yogurt with unsweetened coconut or cashew yogurt, and use neutral oil instead of ghee.Storage: Store in an airtight container in the refrigerator for up to 3 days. Reheat on low with a splash of water. Do not freeze.Make-ahead: Prepare the kadhi base a day ahead and refrigerate. Make the tadka fresh just before serving.Gluten-free note: Check that your asafoetida (hing) is wheat-free — some brands use wheat flour as a filler.