Combine yogurt and 1/2 tsp baking soda in another small bowl and whisk. Add this to the above flour mixture and mix well with a spoon.
Next add water as needed and mix to make a thick batter (thicker than dosa batter). I had to add about 3/4 up water.
Cover and let the batter sit to ferment overnight or for about 7 - 8 hours or lesser time if you live in a hot climate.
In the morning / just before steaming the dhokla, grease a thali or a circular cake tin of about 6- 7 inches in diameter. Keep your steamer / pressure cooker ready for steaming by pouring water in the base
Add salt, asafoetida and green chili to the fermented batter and mix well with a ladle.
Add 1/2 tsp baking soda to 2 - 3 tbsp. of hot water and stir it with a spoon and add it to the batter.
Stir the batter vigorously using a ladle in clockwise motion for few seconds. You would notice the effervescence and the batter turns frothy.
Sprinkle about 1/4 tsp red chili powder over it. Place it in the steamer / pressure cooker and cover it.
Steam on medium flame for about 15 - 20 minutes or until a knife / toothpick inserted at the center comes out clean
Allow it to rest for a couple of minutes and brush it with about a tsp of oil over the surface using a flat surfaced wooden spatula. Cut into squares using the same spatula or a knife.
These dhokla were served with a green chutney, oil and garlic chatny.