Heat oil in a pan and crackle the mustard seeds. Add urad dal and fry, till it turns brown.
Add chopped onion, curry leaves and green chilli and cook till the onion becomes soft, no need to brown it. Add turmeric powder and cook for a min. remove from fire. Let it cool for 10-15 mins. Add the cooked onion mixture, salt and lemon juice to mashed potatoes. Mix well. Make balls of equal size from the potato mixture.
For the coating
Mix together besan, rice flour, chilli powder, asafetida, hot oil, bicarbonate of soda and salt. Add 1/2 - 3/4 cup water and make a thick batter. The consistency of the batter should be medium thick.
Heat oil in a deep and wide pan. When the oil becomes really hot, reduce the flame to low-medium. Dip the poatato balls in the batter and fry in the hot oil. Flip the balls after a min or two. Fry it till it becomes golden brown in colour.
Serve hot with tomato ketchup and chutney
Notes
I mashed the potatoes without any lumps. But if you like a small bite, dont mash the potatoes completely. If you are boiling the potatoes with salt, be careful when adding salt while cooking.You can adjust the consistency of the batter for outer covering to suit your taste requirements. Make sure that the batter sticks to the bonda, after dipping it. If the batter doesnt stay, add some more rice flour and thicken the batter.You can make the filling in advance and keep in fridge. It's better to serve the bondas hot. I made coriander chutney for the bonda.I got 16 small-medium size bondas from the above qty. I deep fried the bondas.