Boil yellow lentils with onion, green chilli, tomatoes, chilli powder, turmeric, and 200ml water for 30 minutes.
Blend drained black lentils and rice with 15ml water to make a thick batter.
Mix chopped onion, chilli, bicarbonate of soda, and salt into the batter.
Fry mustard and cumin seeds in oil until they spit, then add curry leaves and sambhar powder.
Stir the lentil mixture and tamarind juice into the tempered spices and simmer for five minutes.
Heat deep-frying oil to 180°C.
Shape batter into doughnuts on moist cling film, make center holes, and fry until golden.
Drain doughnuts on paper, garnish sambhar with coriander, and serve together.