Put the onion, ginger and garlic in a food processor and blitz to a smooth paste. Heat the oil in a large flameproof casserole dish or pan over a medium heat. Add the onion paste and fry for 8 mins or until lightly golden. Stir in the curry paste and fry for 1 min more. Add the tomatoes, tomato purée, sugar and cardamom pods. Bring to a simmer and cook, covered, for another 10 mins.
Cook the rice following pack instructions.
Scoop the cardamom out of the curry sauce and discard, then blitz with a hand blender, or in a clean food processor. Return to the pan, add the almonds and prawns, and cook for 5 mins. Season to taste and stir through the cream and coriander. Serve with the rice and naan breads, if you like.