1Prepare the green chutney if making from scratch. In a blender, combine 1 cup fresh mint leaves, 1 cup fresh cilantro, 2-3 green chilies, 1 inch ginger, 2 tablespoons lemon juice, 1/2 teaspoon salt, 1/2 teaspoon cumin, and 2-3 tablespoons water. Blend until completely smooth. The chutney should be vibrant green and pack a spicy, herbaceous punch. Taste and adjust salt and chili. Transfer to a squeeze bottle or bowl. Store-bought green chutney works well if you are short on time.
2Prepare the sweet tamarind chutney if making from scratch. In a small saucepan, combine 1/4 cup tamarind paste, 1/4 cup jaggery or brown sugar, 1/2 teaspoon roasted cumin powder, 1/4 teaspoon red chili powder, 1/4 teaspoon black salt, and 1/2 cup water. Simmer over low heat for 8-10 minutes, stirring occasionally, until the chutney thickens and becomes glossy. It should coat a spoon and drizzle in a thick stream. Cool completely - it will thicken further as it cools. Store-bought tamarind chutney is a convenient alternative.
3Prepare the yogurt. Use thick, full-fat yogurt for best results - Greek yogurt works well. Whisk the yogurt until completely smooth with no lumps. If the yogurt is too thick, add 1-2 tablespoons of water or milk to achieve a drizzling consistency. Season lightly with a pinch of salt and sugar to balance. The yogurt should be cool and creamy, providing a soothing contrast to the spices. Keep refrigerated until ready to assemble.
4Prepare the remaining components. Boil the potatoes until tender, then peel and dice into small cubes (about 1/2 inch). Season the potatoes with a pinch of chaat masala and black salt while still warm so they absorb the flavors. If using canned chickpeas, drain and rinse them. If cooking from dried, boil until tender. Season the chickpeas similarly with chaat masala and black salt. Finely dice the onion if using. Have the sev, pomegranate seeds, and fresh herbs ready for garnishing.
5Assemble the papdi chaat just before serving - timing is crucial as the papdi will soften quickly. Arrange the papdi crackers in a single layer on a serving plate or in individual shallow bowls. For a more dramatic presentation, you can break some papdi into smaller pieces and leave others whole. Scatter the diced potatoes and chickpeas evenly over the papdi. If using diced onion, sprinkle it over the top.
6Add the toppings in the proper order for maximum impact. Drizzle the whisked yogurt generously over everything - it should pool in some areas and coat others. Next, drizzle the sweet tamarind chutney in artistic zigzags across the plate. Follow with the spicy green chutney, using less than the tamarind since it is more potent. Sprinkle chaat masala, roasted cumin powder, red chili powder, and black salt over everything. Top with a generous handful of fine sev for crucial crunch, pomegranate seeds for jewel-like bursts of sweetness, and fresh cilantro and mint leaves for color and freshness. Serve immediately - within 2-3 minutes the papdi will begin to soften. Eat with a spoon, making sure each bite includes papdi, filling, yogurt, both chutneys, and sev. The magic of papdi chaat is experiencing all the contrasting flavors and textures in every mouthful.