Prepare the batter by soaking 2 cups of rice and 1 cup of urad dal separately for 4-5 hours. Drain the water and grind them separately into a smooth paste, adding water as needed. Mix the rice and urad dal paste together, add 1 teaspoon of salt, and let it ferment overnight or for 8-10 hours.
Finely chop 2 large onions, 2 green chilies, and a handful of fresh coriander leaves. Mix them together in a bowl and set aside.
Heat a non-stick tawa or griddle over medium heat. Grease it lightly with oil.
Pour a ladleful of the fermented batter onto the tawa and spread it evenly into a thick, round pancake.
Sprinkle the onion mixture generously over the top of the uttapam, and gently press it down with the back of a spoon.
Drizzle a teaspoon of oil around the edges of the uttapam and let it cook for 2-3 minutes until the bottom turns golden brown.
Flip the uttapam carefully and cook the other side for another 2-3 minutes until it turns golden brown.
Remove the uttapam from the tawa and serve hot with coconut chutney and sambar.