In a large bowl add in rava, rice flour, salt, curd and mix well. Now add in water and make a smooth and thick batter out of it, whip the batter for 2 minutes.
Add chopped onion, carrot, capsicum, chilli, coriander leaves, turmeric powder, red chilli flakes and mix well.
Cover and let it rest for 15 minutes, after 15 minutes mix well then add Eno (fruit salt) and mix again. In place of Eno, baking soda can also be used (half the amount of Eno).
Take a sandwich toaster and grease it well with oil. Heat greased toaster on lowest flame and sprinkle in mustard seeds.
Add 2-3 ladle of batter spread evenly and don’t let it overflow. Close the toaster and cook on low flame for 1 minute.
After 1 minute, flip the toaster and cook for 1 minute. Switch the side again and toast sandwich till fully cooked from both the sides.