Heat a large saucepan or 2 litre (3½ pint) flameproof casserole, spray lightly with oil and cook the lamb briefly, in batches if necessary, until brown. Remove the lamb with a slotted spoon, add the onion and carrot to the pan and cook briefly to brown. Return the lamb, stir in all the remaining ingredients and season to taste.
Simmer covered for 1 hour or until the lamb is tender. At the end of the cooking time, stir in the coriander and lime juice.
Meanwhile, cook the couscous according to the instructions on the packet and set aside for 5 minutes. Serve the hot tagine over the couscous and garnish with coriander sprigs.