Preheat the oven to 220°C. Wash and dry the lamb. Using a sharp knife, make slits 3-4cm deep into the flesh. Spread the Saclà Tomato & Garlic Stir Through Sauce over the surface, pushing it down into the slits. Push the garlic slivers into the slits. If time allows, cover and refrigerate for up to 2-3 hours. Brush a roasting tin with a little oil and lay most of the thyme in it. Set the lamb on top, season with salt then drizzle with 2 tbsp of the oil. Roast for 20 minutes. Reduce the heat to 190°C. Cut the heads of garlic in half and place them around the lamb, then drizzle over the remaining oil. Pour in 150ml of the stock and cover with foil. Continue to roast the lamb for a further 1-1½hours. Remove the foil for the last 20-25 minutes of cooking, basting the lamb and garlic.