In a large mixing bowl, combine the sooji and besan. Add simple buttermilk gradually, stirring well with a spoon to produce a smooth batter.
Meanwhile, cut the vegetables finely and season them with salt. Check the consistency of the batter after 20 minutes and add a little water if necessary.
A non-stick pan should be heated over a medium flame. Spread some drops of oil to the surface. Reduce the heat to low and pour a ladleful of batter into a circle, spreading it out evenly.
Chopped onions, tomatoes, chiles, and coriander go on top of the uttapa. Evenly distribute the vegetables. With the help of a spoon, drizzle a few drops of oil over the edge and cover with a lid.
Cook, stirring occasionally, until the top surface turns off white and the bottom side turns light brown.
After 2 minutes, gently rotate it and cook for 1 minute on the other side. Serve hot with tomato ketchup or your favourite chutney.