Weigh the lamb and calculate the cooking time. Preheat the oven to 200°C, fan 180°C, gas mark 6.
Lay the leeks and halved onions over the base of a roasting tin. Rub the lamb with olive oil and sprinkle with the herbs. Season well and lift onto the vegetables.
Roast for 30 minutes. Reduce the heat to 170°C, fan 150°C, gas mark 5 and cook for a further 15 minutes per 500g. Add the garlic bulbs to the roasting tin.
Remove the joint from the oven and lift onto a carving platter. Cover with foil and a clean tea towel and leave to rest.
Remove the cooked vegetables and garlic from the roasting tin and drain off any fatty residues. Stir a little flour into the tin and gradually add water or stock and simmer over a low heat, stirring well until thickened to make a gravy.
Serve slices of the lamb with the gravy and roasted garlic bulbs, as well as your choice of seasonal vegetables.
Cook's tip: The garlic takes on a fantastic sweet flavour when roasted like this. You can squeeze out the individual cloves and eat with the roast, or if you're not such a garlic fan, they just add a fantastic aroma and flavour to the roasted meat during cooking. They also look quite decorative on the platter too!