Whisk together the yogurt, besan, turmeric powder and red chilli powder to a smooth batter.
Heat the oil in a kadhai and add the asafetida to it. Over medium heat, pour in the yogurt batter and keep whisking. Whisking ensures that the sudden heat does not separate the yogurt.
Keep cooking it on medium low heat and whisking every minute or so, till small bubbles start forming on top.
Add water to thin the curry out and keep cooking it, whisking every now and then, for about 30 mins.
Now add the chopped spinach and salt, mix well and cook for 5 mins or so, till the spinach wilts.