Begin by preparing the dosa batter if not already done. You may also use leftover batter for convenience. In a separate mixing bowl, combine the finely chopped onions, tomatoes, green chilies, and coriander leaves. Mix well to create the topping mixture.
Heat a flat skillet, non-stick tava, or cast-iron griddle over medium heat until it’s evenly hot. Pour a ladleful of dosa batter onto the surface and gently spread it into a thick circular disc, similar to a dosa but slightly smaller in diameter and thicker in depth.
Generously spoon the prepared onion-tomato-chili mixture over the surface of the batter. Using a spatula, lightly press the toppings into the batter so that they adhere well and cook evenly.
Sprinkle a pinch of sambar powder, pav bhaji masala, or idli podi over the surface, along with a bit of salt if desired. Drizzle oil around the edges and a few drops over the top to help with browning and flavor.
Allow the base to cook until it turns crisp and golden brown. Flip the uttapam carefully with a spatula and drizzle a little more oil on the newly exposed side. Let it cook until the vegetables on top are slightly caramelized and tender.
If the first side hasn’t browned to your preference, flip once more and cook briefly. Once both sides are evenly cooked and the surface is golden with a slight crisp, remove from heat.
Serve the hot Tomato Uttapam immediately alongside coconut chutney and a bowl of piping hot sambar. It also pairs wonderfully with curd chutney or a side of sweetened or plain yogurt. Best enjoyed fresh off the griddle for maximum flavor and texture.