Make the spice paste by placing the cumin seeds, dried chillies, cardamom seeds, cinnamon stick, peppercorns, garlic and vinegar into a small food processor and processing into a smooth paste. Place the pork in a non-metallic dish and pour over the paste. Rub into the pork, cover and marinate in the refrigerator for up to 24 hours.
Heat the oil in a large, heavy-based saucepan and add the onion. Stir-fry for 3–4 minutes, then add the curry powder and the pork. Stir-fry for 3–4 minutes and then stir in the potatoes, tomato purée, sugar, chopped tomatoes and stock.
Season well and bring to the boil. Cover tightly and reduce the heat to low. Gently simmer for 1½ hours or until the pork is tender.
Garnish with chopped fresh coriander and serve immediately with steamed white rice.