Heat 1 tbsp oil in a non stick pan (I prefer mustard oil to get the authentic taste).
Add mustard seeds, let it brown and splutter, add the grated onion, ginger and chopped green chili and fry till the raw smell goes off.
Add the mashed potato, chopped coriander and salt. Mix on a low flame. Keep aside and let it cool.
Prepare the batter by adding all the ingredients said above and mix well so that no lumps remain.
Be sure to make the batter a little thick so that it coats the stuffing nicely.
Heat the oil for deep frying. Divide the potato mixture into equal lemon sized balls and flatten them with your palm.
Dip each flattened piece in the batter and deep fry until golden brown. Make sure you have 1 inch of oil for deep frying in the pan.
Serve hot with chutney or ketchup.
Notes
You may prefer not to do Step 3 above depending upon your choice. I prefer to follow the above said method so that there is a tinge of fried taste to the mashed potatoes.