In a medium skillet over medium heat, fry the cilantro, garlic, and green chilies until fragrant, about 1–2 minutes. Add the salt, mustard seeds, and turmeric, stirring for 30 seconds. Add the mashed potatoes and mix until fully combined. Shape the mixture into evenly sized balls, roughly 2 inches/5 cm in diameter, and refrigerate for 20–30 minutes to firm up.
For the chutney, rinse the cilantro and mint well, discarding thick stems. In a blender, combine the herbs, chilies, garlic, ginger, salt, roasted cumin powder, chaat masala, black salt, and lemon juice. Blend until smooth, adding water gradually to reach a thick but pourable consistency. Taste and adjust seasoning — add more lemon juice for tang, chili for heat, or cumin for smokiness.
In a separate mixing bowl, whisk together the chickpea flour, egg, salt, turmeric, and water until smooth and lump-free.
Heat oil in a deep-frying pan to 355°F/180°C.
Coat each potato ball in the batter, then carefully lower into the hot oil. Fry in batches for 4–5 minutes, turning occasionally, until golden and crisp. Remove with a slotted spoon and drain on paper towels. Serve warm.