Wash and soak channa dal for 2 to 3 hrs. Drain the water completely and keep aside. Grind together with other ingredients listed under "To grind" with 1 - 2 teaspoon of water into a coarse mixture (less water helps in crispy vadai texture. Avoid using too much water)
Transfer the mixture to the mixing bowl and add onions, curry leaves, coriander leaves and mix well
Preheat the oven to 400 degree Fahrenhait. Take the baking tray and brush the oil
Take small portions of the mixture, and flatten them using your palm (thinner vadai yields crispy vadai)
Place them in the greased tray and brush the top portion of each vadai with little oil to avoid drying out and of course for the crispy finish
Bake the vadai for 11 - 15 min, till they have browned the underside (adjust time according to your oven) . Carefully flip them and bake again for another 11 - 15 minutes. Remove and serve immediately. Serve with chutney/ ketchup. I generally love to finish them without any sides