In a medium bowl, combine the finely chopped red onion, chopped tomato, minced green chilies, and fresh coriander leaves. Toss well to mix the ingredients evenly. Set aside.

Heat a tawa, flat cast iron griddle, or heavy-bottomed skillet over medium heat. To test if it is ready, sprinkle a few drops of water on the surface; the water should sizzle and evaporate immediately. Wipe the surface clean with a damp paper towel.

Reduce the heat to medium-low. Pour a large ladleful (about 1 cup) of the fermented batter onto the center of the hot griddle. Using the back of the ladle, gently spread the batter outward in a circular motion to form a thick pancake, about 15 cm / 6 inches in diameter. Do not spread it thin like a dosa; it should be about 1 cm thick.

Immediately sprinkle about 3 to 4 tablespoons of the prepared vegetable mixture evenly over the surface of the wet batter. If using salt, sprinkle a tiny pinch of salt over the toppings. Using the flat side of your spatula, gently press the toppings down so they embed firmly into the batter.

Drizzle 1 teaspoon of oil or ghee around the edges of the uttapam and a few drops over the toppings. Cover the pan with a lid. Cook on medium-low heat for 3 to 4 minutes. You will see bubbles forming and popping on the surface, and the edges will start to turn a light golden-brown and release from the pan.

Remove the lid. Slide the spatula underneath and carefully flip the uttapam over. Press down gently with the spatula so the vegetables make flat contact with the hot griddle. Cook uncovered for 1 to 2 minutes on this side, until the vegetables are slightly caramelized and golden brown.

Flip the uttapam back once more to ensure the bottom is fully cooked and crisp. Remove it from the griddle and place it on a serving plate. Repeat the process for the remaining batter and toppings, cooking each uttapam for about 5 minutes total and wiping the griddle clean between batches.