Soak the lentils in a large bowl of water for 1 hour.
Drain the lentils well and transfer to a blender or food processor.
Process, gradually adding about 2 tablespoons water, until thick.
Transfer the batter to a clean bowl.
Mix in the flour, vegetables, ginger, chillies, coriander, curry leaves and roasted chana dal.
Season with salt.
Heat enough vegetable oil for deep-frying in a kadhai or deep, heavy-based pan to i8o‘C/35o‘F, or until a cube of bread browns in 30 seconds.
Then reduce the heat slightly so that the vadai will not brown too quickly.
Working in batches, take a ladleful of the mixture and, using damp hands, flatten it into a 5 cm / 2 inch round patty.
Gently lower the patty into the hot oil.
Deep-fry, turning carefully, for about 2-3 minutes, or until golden brown and crisp.
Remove with a slotted spoon.
Drain and on kitchen paper (paper towels).
Repeat until all the mixture is used up.