To prepare the batter for Dhokla, clean, wash and soak rice, and lentils in water for 6 hours or overnight.
Grind the soaked rice, lentils along with green chillies, ginger using curd to blend the mixture. If the batter is too thick than 2 to 3 tbsp of water can be used to grind it.
The consistency of the batter should be such that when you use a ladle to pour it does not pour itself but falls in dollops. Stir in the salt and allow the mixture to ferment in warm place for 8 hours or maybe overnight. Fermenting time usually depends on the weather outside. During summer season it takes less time than winters.
Once fermented you will notice the dough has increased in volume and a sour aroma will release once you bring out the batter. Gently stir in the 1 tbsp oil.
Grease one flat plate with oil all round the sides and the base.
To heat Tupperware Steamer, fill wide large pan with water and put it over medium to high heat. Once water starts simmering arrange Tupperware Steamer over it (as directed in the instruction manual).
Add fruit salt to the batter and stir to combine evenly in batter. Immediately pour in batter to the greased plate. Leven the batter with the back of a spoon.
Place the dhokla plate in steamer and steam for about 10 minutes on high heat, until the Dhokla is cooked that when a tested inserted into the center comes out clean.
Remove Dhokla from the steam, and let it sit for 10 minutes before tempering.
For tempering, heat oil in a small pan on medium heat; add the mustard seeds, curry leaves, asafoetida and allow them to crackle. Then add turmeric and red chili powder fry for 1 – 2 second. Turn off the heat.
Pour the seasoning over the Khatta Dhokla.
Cut the Khatta Dhokla into desired shapes and serve with Green Chutney.