Saute mustard seeds and curry leaves in hot oil, in a large skillet until they start to sizzle. Add in chopped onions and fry for a minute or two until soft and lightly brown.
Add in spices and salt, and fry for a few seconds until fragrant.
Add in mashed potato and mix everything well. Add in fresh coriander leaves and mix again. Set aside to cool for a bit, until it is easy to handle by hand.
In a large bowl, add in chickpea flour and a pinch of salt. Slowly add in water and whisk to form a thick, pancake-like batter.
Heat enough oil in a deep wok for deep frying. Divide the cooled potato filling into golf ball sized portions.
Gently coat each potato ball into the chickpea batter and carefully add it into the hot oil in batches. Try and not crowd the wok by adding too many at a time.
Deep fry for 6-8 minutes on medium heat, turning them over to ensure that they brown and cook on all sides. Once the fritters are golden brown and crisp, remove them on paper towels to soak up any excess oil.