Put half of the rice flour into a pan and add 1 tablespoon water. Bring to the boil, then remove from the heat and mix with the remaining half of the rice flour. Set aside for 24 hours.
Soak the urad dal in water for 4 hours. Soak the chana dal in another bowl of water for 1 hour.
Drain most of the water from the urad dal, transfer to a blender or food processor, and process to make a fine paste.
Transfer to a bowl, add the rice flour, curry leaves, onions, chilies, ginger, and season with salt. Mix together, then add the chana dal and mix well.
Heat 1 tablespoon oil for tempering in a small, heavy-based frying pan (skillet), add the mustard seeds, chilies, 1/2 teaspoon urad dal, asafoetida, and stir-fry for about 1 minute, or until dark brown.
Remove from the heat and add to the rice flour mixture. Mix well, then shape the mixture into 5-cm / 2-inch rounds about 2 cm / 1/4 inch thick.
Heat the oil for deep-frying in a kadhai or deep, heavy-based pan to 180’C / 350'F, or until a cube of bread browns in 30 seconds.
Working in batches, deep-fry the rounds for about 2-3 minutes, or until golden brown. Remove with a slotted spoon and drain on kitchen paper. Allow to cool.
This snack will keep for several days in an airtight container.