Cut the lamb on the fatty side. Make incisions of around a centimeter deep in diamant shape. Remove the needles from a few rosemary twigs and thyme and cut them finely. Chop a few of the garlic cloves aas well. Rub the lamb well with olive oil and the herb mixture. Make sure you press it inside the cuts. Sprinkle liberally with salt and pepper.
Preheat the oven on full temperature (usually around 230˚C) Take the rest of the garlic apart into separate cloves and smash them on the side with a large knife. Spread the onion, a part of the garlic, thyme and rosemary across the bottom of a baking tray. Place the leg of lamb on top and sprinkle with the rest of the garlic and the herbs. Place in the oven and lower the temperature immediately to 150˚C.
Use the thermometer as that is the only way to be sure that your meat has reached the desired temperature. Medium pink is around 60˚C. You can keep it in the oven longer but never go further than 70˚C. Remember that it will continue cooking once out of the oven too!
Roast the lamb for about four hours in the oven and check regularly to make sure the top doesn't get too brown. Baste it from time to time with the cooking liquids in the oventray. If it gets too dark but is not cooked properly yet, cover the baking tray with aluminium foil and place back in the oven
Take it out of the oven once it reaches the right temperature and wrap in aluminiumfoil. Place a towel over that to keep it warm and leave to rest at least for half an hour in a warm spot. Cut at the table with a large and sharp knife and serve with roasted vegetables and baked potatoes