Cook the potatoes together with the turmeric in a large pan of boiling water for about 15 minutes, or until tender. Drain and set aside.
Put the poppy seeds, ginger, garlic, cashew nuts, and coconut in a mortar and pound with a pestle or a spice grinder and process until ground.
Heat the oil or ghee in a heavy-based pan over medium heat, add the whole spices and chillies and lightly fry for about 1-2 minutes.
Add the ground spices and ground coriander and fry for about 1 minute, or until fragrant.
Add the tomatoes together with the boiled potatoes to the pan and cook for a few minutes. Season with salt, then add the chilli powder and yoghurt. Reduce the heat to low and simmer for a few minutes.
Add the chopped coriander and remove the pan from the heat. Stir in the lime juice and serve.