Combine the rice and lentils in a large bowl.
Pour in enough water to cover and allow to soak in water for 3-4 hours.
Drain the rice and lentils.
Transfer to a blender or food processor and process, gradually adding 500ml / 18fl oz (2 1/4 cups) water to make to a smooth, stiff batter.
Heat the 1 tablespoon oil for the tempering in a small, heavy-based frying pan (skillet).
Add the mustard seeds and stir-fry for about 1 minute, or until they start spluttering, then add the curry leaves and onions and stir-fry for about 2 minutes.
Stir in the coconut and fry for a further 2 minutes, then put the contents of the pan into the batter.
Season with salt and stir in the coriander leaves.
Heat enough oil for deep-frying in a kadhai or deep frying pan (skillet) to 180’C / 350°F, or until a cube of bread browns in 30 seconds, then reduce the heat slightly.
Working in batches, shape the batter into small lime-sized balls, then slip them gently into the hot oil.
Deep-fry, turning frequently for about 1-2 minutes, or until golden brown and crisp.
Remove with a slotted spoon and drain on kitchen paper (paper towels).