Wash the rice and lentils separately in several changes of water. Leave each to soak in a large pan of clean water for a couple of hours
Drain the rice and lentils, place in a blender, and grind to a smooth paste. Add a little water to form a thick batter
Add the salt, sugar and baking soda to the batter and mix well. Set aside to ferment at room temperature for at least 4 hours and up to 15
Place the chopped onion, diced tomatoes, chopped green chile, and chopped cilantro in separate bowls
Heat a griddle or nonstick pan. Brush with a little ghee or oil and pour in 2 to 3 tablespoons of the batter. Spread evenly with the back of a spoon into a thick circle 4 inches in diameter.
Garnish the surface with a little each of the onion, tomato, green chile, and chopped cilantro and spread evenly.
Sprinkle some of the oil or ghee over and around the bread. Cook over low heat until small bubbles appear on the surface, about 5 minutes
Turn over and cook the other side until crisp and golden. Alternatively, you can finish it off under a hot broiler for 1 to 2 minutes. Serve hot with South Indian lentil curry
Sambhar (South Indian Lentil Curry):
Heat half the oil in a pan and add the red chiles, coriander seeds, fenugreek, and grated coconut. Cook, stirring, for 5 to 10 minutes; set aside to cool
Wash the yellow lentils in several changes of water and place in a large pan of water. Bring to a boil, add the ground turmeric, and simmer for 15 to 20 minutes
Put the coconut, chile and spice mix in a food processor or blender and process to a fine paste.
When the lentils are soft and mushy add the sliced onions, asafetida, diced tomatoes, diced eggplant, and coconut paste and continue to cook for 5 minutes. Add the tamarind pulp, salt, to taste, and sugar and simmer for 2 minutes longer
Heat the remaining oil in a separate pan and add the curry leaves and mustard seeds. When they begin to crackle, pour them over the lentils and serve immediately with plain boiled rice.