Peel onions and potatoes and cut them into 2-3 mm thick rounds and keep them ready in a plate.
Add besan, rice flour, turmeric powder, red chilli powder, ajwain, baking soda and salt in a mixing bowl.
Slowly start adding some water and make a smooth batter. Do not make the batter runny or you will not be able to quote it on the sliced vegetables.
Heat oil in a frying pan. To check if oil is ready, add a drop of batter to the hot oil. If it puffs up right away, oil is ready for frying fritters.
Dip sliced potatoes in the batter and fry them in the oil till they turn golden on both sides. Next fry sliced onions dipped in the batter till they turn golden brown.
Serve it hot with some tomato ketchup or Mint chutney.