Arrange the puri shells on a serving platter. Gently tap the top center of each shell with your thumb to create a hole big enough for the filling.
In a bowl, combine the boiled mashed potatoes, chopped red onions, salt to taste, and a pinch of chaat masala. Mix well.
Fill each puri shell halfway with the potato and onion mixture.
Add half a teaspoon of sweet tamarind chutney and a quarter teaspoon of spicy green chutney into each stuffed puri.
Whisk the cold yogurt with sugar and a pinch of salt until silky. Spoon the sweet yogurt generously over each puri until they are filled to the brim.
Sprinkle roasted cumin powder, red chili powder, and chaat masala over the yogurt. Top with a generous amount of nylon sev, fresh coriander, and serve immediately.