Soak the chana dal in a bowl of water overnight.
Drain the dal, then place in a blender or food processor and process until ground and a soft dropping consistency.
Oil a 1.5-litre / 2 1/2-pint (6 1/4-cup) round container, such as a cake tin (pan), that fits inside a steamer, then place the tin inside the steamer and heat.
If you don't have a steamer, use a large saucepan or Dutchoven with a lid and place a trivet or upside-down plate in the base.
Pour in enough hot water to come halfway up the side of the cake tin. Place over medium heat.
Put the ground dal, ginger, lemon juice, chillies, sugar, bicarbonate of soda (baking soda), 3 tablespoons oil and salt to taste in a large bowl, add 250ml / 18fl oz (1 cup) lukewarm water and mix quickly.
Pour the batter into the heated tin. It should be just over half full, then carefully cover with a clean cloth or a round of baking paper (parchment) and put on the lid.
The steamer should remain sealed. Steam for about 50-60 minutes, or until done. It will rise and be firm to the touch.
Carefully remove the tin from the steamer and let stand for 15 minutes.
Leave the dhokla (cake) in the container. Cut intowedges while still warm.
For the tempering, heat the remaining oil ina heavy-based pan, then when the oil is hot, reducethe heat, add the curry leaves and stir-fry for 1 minute.
Add the green chilies and black mustard seeds and stir-fry for about 2 minutes, or until the spices start to crackle.
Add 125ml / 4 1/2fl oz (1/2 cup) water and pour the mixture on top of the dhokla. Serve with chutney.