Heat the oil in a large flameproof casserole dish then add the lamb and sear it on all sides. Add the onions and carrots and continue to cook over a medium heat, stirring regularly, until the onions have softened.
Add the cumin and coriander seeds and season generously with salt and pepper. Pour in some of the stock or water to cover, bring to the boil then reduce the heat and leave to simmer for about an hour until the meat is tender.
While the lamb is cooking rinse the rice until the water runs clear, then leave it to soak in warm water for 30 minutes.
Using a slotted spoon remove the lamb and vegetables from the casserole dish. Pour the remaining liquid into a jug and make it up to 800 ml with stock or water. Put the meat and vegetables back into the dish.
Drain the rice thoroughly and add it to the dish. Stir to coat in the juices, then pour in the 800 ml stock.
Peel away the outer layers of skin from the head of garlic and trim the top with a sharp knife. Put the garlic head in the centre of the rice.
Bring to the boil then reduce the heat and cover with a lid. Simmer for 15 minutes until the rice has cooked and absorbed all the liquid. Remove the dish from the heat, take off the lid and cover with a clean folded tea towel then replace the lid. Leave to steam off the heat for ten minutes.
Fluff up the rice a little and serve the dish from the casserole with a few gherkins and chillies if you like.