To make the stuffing, place your mashed potatoes on a wide tray or big plate and spread the potatoes out. Now, heat oil in a frying pan. Once hot add the mustard seeds. When they begin to crackle, add the curry leaves as well as the onions and mix.
Cook the onions for a minute, then add the ginger and garlic. Continue to cook for 2-3 minutes on a medium heat until light golden in colour then add to the potatoes. (If you find the pan looks dry during the cooking process, you can add a little more oil or a splash of water to ensure nothing burns).
Now add the salt and remaining spices as well as the chillies and fresh coriander. Mix well using your hand. Taste for salt and chilli and tweak according to your personal taste preference. Once combined, roll into balls. You can vary the size on your personal preference! Traditionally they are a little bigger than golf balls. Once rolled, keep aside.
Fill a small wok or saucepan with 4-5cms of oil. Place on your cooker on a low heat setting.
To create the batter, add 1/4 tsp of salt, 1/4tsp turmeric powder and a pinch of chilli powder to the sifted gram flour. Gradually add the water and whisk simultaneously. Add enough water so you have a thick, smooth, lump free batter. It should easily coat the back of a spoon but should be a little thicker than pancake batter.
Increase the temperature of the oil in the wok to medium. To check if the oil is hot enough, add a little spoon of batter to the pan. It should float up to the surface and bubble within about 5 seconds. (If it is too cold it will stay at the bottom and if it is too hot it will come up to the surface immediately and turn brown).
Once you're happy with the oil temperature, place a potato ball in the batter. Spoon over the batter so the ball is completely covered in it. Now, pick up the bowl containing the batter and hold it close to the wok. Use the spoon to carefully transfer the battered vada into the oil. Repeat until you wok is full (being careful not to overcrowd).
After the vadas are sealed (take 10-12 seconds), use a slotted spoon to keep them moving. Cook on a medium heat for 4-6 minutes until golden and crisp to touch. They should stay yellowish - light golden in the 2-3 minutes and then eventually begin to turn completely golden in the final 2-3 minutes. Keep them moving in the oil to prevent any dark spots.
Once ready, use your slotted spoon to remove the vadas from the pan - draining as much oil as possible before placing them on some kitchen paper.
Repeat with the remaining potato balls. Serve with your favourite dipping sauces and chutneys!