Heat the oil in a pan. Add chopped onions and garlic and fry for 5-7 minutes until they become brown.
While the onions are browning, heat a separate pan and add the cumin and coriander seeds, cardamoms and peppercorns. Then dry fry for a couple of minutes.
Take the pan off the heat and set aside to cool. Grind the whole spices in a pestle and mortar or a coffee grinder.
In the pan of onions, tip in the tomatoes and mix for a minute.
Add the ground spices along with the turmeric, salt, chili and ginger. Mix thoroughly and stir in the prawns and then the potatoes.
Cook for 2-3 minutes and serve with crusty white bread or a crisp green salad.