Soak all the lentils and rice overnight with enough water.(4 hours minimum)
Blend with minimum water & salt to get a thick batter,keep it aside for 6 hrs/overnight.
Add ginger,chillies,lemon juice and salt to the batter,get the steamer ready with water.
Add the baking soda to the mix, mix it gently.
Transfer the batter to a steaming plate(pre greased with oil)
Steam for 15-20 minutes or until cooked. Once its has cooled after an hour, cut into squares.
Meanwhile, prepare the tempering / tadka in a pan.Heat oil ,add mustard seeds, curry leaves and chilies. Add ¼ cup water at the end.
Add the dhokla pieces, mix gently and cover it. Cook on low flame for 2 minutes, turn off the heat and let it sit for few minutes. Garnish with coriander, serve with green chutney.
Notes
The addition of water in the tempering with make the dhokla more soft