Preheat the oven to 220°C fan.
Boil the potatoes for 6 minutes until almost cooked, then roast with oil, chaat masala, turmeric, salt, and pepper.
Make the coriander chutney by blending all the ingredients until smooth.
Whisk together the tamarind dressing ingredients with water.
Spread yoghurt on a serving platter, top with coriander chutney, tamarind dressing, potatoes, onion, chilli, and seeds.
Drizzle with remaining tamarind dressing and serve.