Cook the potatoes in a large saucepan of boiling, salted water for 10 minutes until just starting to turn soft. Drain and allow to cool.
Heat the oil in a large saucepan over a medium heat and fry the cumin seeds and black poppy seeds for 1 minute. Add the onion, garlic and ginger and fry for 2-3 minutes, stirring occasionally. Add the remaining spices, salt and sugar and continue to cook for 1-2 minutes.
Add the potato chunks and water and bring to a simmer. Cover and cook on a low heat for 15-20 minutes until soft and just starting to disintegrate.
Adjust the seasoning if necessary and serve in a terracotta dish.