Take some oil in a pan, let it heat up on medium heat. Once hot, add mustard seeds, let them splutter.
As soon as they splutter, add the curry leaves, ginger, chilies. Stir. Add chopped onions too.
Mix it well, let onions cook a little bit and add urad dal or chana dal, cashewnuts. Followed by the turmeric and salt.
Once the onions look cooked, add in the chopped boiled potatoes. Roughly, mash it with the back of the spoon. Do not completely mash it, leave some chunks. Mix well to combine everything.
Turn off the heat, add lemon juice and cilantro. Again, mix it well.
Let it cool down, divide it in equal portions. I got 10 tennis ball sized rounds out of it. You can remove the curry leaves at this point before rolling them into balls.
Heat up some oil for deep frying on low-medium heat.
While the oil heats up, take dosa batter in a mixing bowl, add baking soda or Eno, whatever you are using. Stir it well to mix the soda evenly in the batter.
Once the oil is hot around 250-300 F. (Oil should not be too hot). Dip the potato balls with the help of a spoon into dosa batter and drop them carefully into the oil and fry them until golden crispy on all sides.
Repeat, until all the potato mixture is fried.
Serve it hot/warm with coconut cilantro chutney.