You can use either Idli or Dosa batter to make Oothapam
Add water and dilute it such that it comes to good dropping consistency. It should be a little more watery than dosa batter. Pour it on a hot griddle and spread it to a 4' diameter and 1/4' thick. Don't spread it like a dosa and make it thin.
Add the topping of your choice as suggested below.
Add oil and roast till golden brown on one side.
Flip it and roast on the other side with a tsp of oil till golden brown.
Toss it into a plate and serve with chutney and sambhar.
Onion oothapam topping: mix together chopped onions, coriander leaves and green chillies. Use about 3-4 tbsp of topping on each oothapam.
Mixed vegetable topping: Mix together grated carrot, finely chopped capsicum (bell pepper), onions, green chillies, cilantro, tomatoes. Use about 3-4 tbsp of topping