Preheat the oven to 180°C, fan 160°C, gas 4. Heat the oil in a large flameproof casserole on the hob and fry the lamb in batches, until browned all over. Remove from the casserole using a slotted spoon and set aside.
Add the onion to the casserole and cook for about 5 mins until really softened, then add the chopped garlic and rosemary leaves and cook for a further min. Add the carrots, parsnips and lentils and stir well. Add the tomato purée and cook for a further min.
Return the lamb to the casserole with the chopped tomatoes, stock and apricots and stir well.
Cover with a lid and cook in the oven for 1 hour until the lamb is meltingly tender and the lentils are completely cooked to form a thickened sauce.
Meanwhile, prepare the couscous according to the pack instructions.
Ladle the casserole onto plates and serve with the couscous, a sprinkling of flat-leaf parsley and a teaspoon of harissa paste for extra spice, if you like.