In a mixing bowl, combine the flour, onion, curry leaves, green chilies (if using) and salt. Add about ½~¾ cup of water to make a very thick paste like batter. Add water very slowly, so that you won’t end up with watery batter.
Cover the bowl and set aside for 2~3 hours.
Just before making the vadas, add the baking soda and mix well.
Heat oil for deep frying. Take lemon size ball of the batter and place it on a damp plastic ziploc bag or your palm. Flatten it slightly and poke a hole right in the middle (as in a donut or bagel). Gently slide the vada into the hot oil and fry on medium flame until golden. Flip carefully and fry on the second size until golden. Fry 3~4 vadas at a time depending on the size of your pan. Repeat with rest of the batter.
Remove using a slotted spoon onto a paper towel lined plate and serve hot with chutney.
** I added onion since I made them for our evening snack, but if you are making them for the festival, then skip the onion all together or add shredded cabbage instead.