Pick over the rice and dal to remove any pebbles or dirt. Process in a food processor or blender until it becomes a coarse flour mixture, about 5 minutes.
Combine this mixture with water, yogurt, and salt in a bowl. Cover it with plastic wrap and set aside in a warm place at least 12 hours or as long as 24. When ready, the batter should be smooth and foamy, a bit thinner than pancake batter.
Just before cooking, add the tomatoes, serrano chiles, red onion, and cilantro. Stir to combine.
Heat 1 teaspoon oil in a small, well-seasoned cast iron skillet over high heat until it is very hot. Pour in 1/3 cup batter, reduce the heat to low, and spread the batter evenly with a ladle or spatula. Fry until well browned, about 2 to 3 minutes per side. Repeat this procedure, heating an additional teaspoon of oil each time, until all the batter is fried. Stack the pancakes between layers of aluminum foil in a roasting pan and keep warm in a 200 degree F oven.
To serve, arrange 2 pancakes on each plate and top with some yogurt, tomatoes, and cilantro. For smaller servings, slice each pancake into quarters and serve with garnishes for dipping.
* Dal is the Indian name for the inside of any whole, dried bean. Urid dal, black bean dal, and orange dal are three varieties available in Indian or Middle Eastern markets. Lentils may be substituted.