In a large bowl, mix rice flour, salt, turmeric powder, and enough water to make a smooth batter. The consistency should be slightly thick but pourable.
Stir in cumin seeds, mustard seeds, chopped onions, tomatoes, green chilies, and coriander leaves. Mix well.
Heat a non-stick or cast-iron skillet over medium heat. Lightly grease it with oil.
Pour a ladleful of batter onto the hot skillet and spread it into a thin, even layer using the back of the ladle or a spatula.
Cook for 2-3 minutes or until the edges start to lift and the bottom is golden brown. You can cover the pan with a lid to help it cook more evenly.
Flip carefully and cook for another 1-2 minutes on the other side until golden and cooked through.
Remove the Chatamari from the pan and place it on a serving plate.
Enjoy your meal!