To make the potatoes, preheat the oven to 220C/200C Fan/Gas 7. Put the potatoes and 2 teaspoons of salt in a saucepan and top with enough cold water to cover by about 4cm/1½in. Place over a medium–high heat, bring to the boil and simmer for 6 minutes, until the potatoes are almost cooked through but still retain a bite. Drain and pat dry.
Line a large baking tray with baking paper and add the potatoes. Add the oil, chaat masala, turmeric, ⅓ teaspoon of salt and a good grind of pepper. Toss to mix thoroughly. Roast in the oven, stirring once or twice, for 35 minutes, or until deeply golden.
Meanwhile, make the coriander chutney. Put all the ingredients and ¼ teaspoon of salt into a food processor or blender and blitz until smooth. Set aside.
To make the tamarind dressing, whisk together all the ingredients in a small bowl with 1½ teaspoons of water until the sugar is dissolved. Set aside.
Spread the yoghurt on a large round serving platter. Top with the coriander chutney, swirling it through without completely incorporating. Drizzle with half of the tamarind dressing, and top with the potatoes, onion and chilli. Drizzle over the remaining tamarind dressing, then sprinkle over the seeds and serve.
