In a large bowl, whisk together yogurt and besan until completely smooth with no lumps.
Add 3 cups of water gradually while whisking to create a thin, lump free mixture.
Pour this mixture into a heavy bottomed pan or kadhai and place it on low to medium heat.
Add turmeric powder, ginger paste, green chilies, jaggery, and salt. Stir well to combine.
Keep stirring continuously for the first 5 to 7 minutes as the kadhi warms up. This prevents curdling.
Once the kadhi starts to simmer gently, reduce the heat to low and let it cook for 12 to 15 minutes. Stir occasionally.
The kadhi will thicken slightly and become smooth. Taste and adjust jaggery or salt if needed.
In a small tadka pan, heat oil or ghee on medium heat.
Add mustard seeds and let them splutter. Then add cumin seeds, asafoetida, curry leaves, and broken red chilies.
Pour this tempering over the kadhi immediately. Stir gently to mix.
Let the kadhi simmer for another 2 minutes so the tempering flavors infuse. Turn off the heat.
Let the kadhi rest for 5 to 10 minutes before serving. It thickens slightly as it cools.
Notes
Equipment - Use a heavy bottomed pan or kadhai to prevent the kadhi from sticking or burning at the bottom. A whisk is essential for a lump free mixture.Make-ahead - Kadhi can be made a day ahead and stored in the fridge. Reheat gently on low heat, adding a splash of water if it has thickened too much.Consistency tip - If your kadhi becomes too thick after resting, just add a little warm water and stir. Gujarati kadhi should be pourable, not paste like.Sweetness level - Adjust jaggery based on your preference. Some families like it sweeter, others prefer just a hint of sweetness.Yogurt quality - Slightly sour yogurt gives the best flavor. If your yogurt is too fresh and mild, add a small squeeze of lemon juice.Serving temperature - Kadhi is best served warm, not piping hot. It is easier on the stomach and the flavors are more balanced.Leftovers - Store in an airtight container in the fridge for up to 3 days. The kadhi may thicken, so thin it out with water when reheating.