Put the onion and garlic in a food processor and process until finely minced. Heat half the oil in a large, deep frying pan over a medium heat, add the cinnamon and cardamom and fry for 1 minute until just beginning to sizzle.
Add the rest of the oil to the pan and stir in the minced onion mixture. Fry over a low heat for 5-6 minutes, stirring occasionally. Stir in the korma paste and cook for a further 1 minute.
Add the chicken to the pan and cook, stirring, over a medium heat for 2-3 minutes until no longer pink. Add nearly all the yogurt, one spoonful at a time, stirring thoroughly after each addition (reserve about 2 tbsp to serve).
Partially cover the pan and simmer gently, stirring occasionally, for 18-20 minutes until the chicken is tender and cooked through. Season to taste.
Meanwhile, cook the basmati rice following the pack instructions. Serve the korma and rice topped with the reserved yogurt and garnished with coriander leaves.