Whisk besan/gram flour and curd in a large bowl until it is smooth and lump-free.
Add around 2 cups of water and whisk well. If you still see some lumps use hand blender to blend well.
Now in a thick bottom vessel heat oil, add methi seeds. Saute for few seconds or until they get a slight golden color.
Next, add mustard seeds and let them splutter. When it start crackle, add minced green chili-ginger. Saute for a while and next add minced garlic.
On a low flame keep sauteing till the raw aroma of ginger, chilli and garlic goes away.
Now add turmeric powder, coriander powder and red chili powder. Saute well.
Now add sliced onion with pinch of salt. When the onions turn translucent and acquire pink color, add the curd-besan mixture to this. Keep stirring it constantly on a low heat.
Make sure to stir the kadhi continuously until it comes to the first boil.
You can then lower the heat after that and and let it cook. You don’t need to stir it all the time after that.
After a while add salt and let it boil for 20-25 minutes.
In between if you feel the kadhi is turning thick, add hot water.
After 25 minutes, kadhi would have considerably thickened and completely cooked.
You can add required amount of hot water here if kadhi looks too thick to you.