in a large mixing bowl, add gram flour (kadalai maavu), rice flour(arisi maavu), turmeric powder, chilli powder, ajwain(carom seeds), salt and cooking/baking soda a pinch(optional).
mix well and add water little at a time to make a thick batter. the batter should not be runny as well as thick . when dropped from a ladle, it should flow like a ribbon.
slice the onions and keep it ready. dip the onions slices and evenly coat it on both sides with batter. take it out and drop it in oil.
before adding it to oil, check if the oil is hot by dropping a pinch of batter into it and if it sizzles and gets brown faster, oil is hot and perfect. add the onions to the hot oil.
keep turning sides until both sides are evenly brown.
serve hot with chai.