Remove your lamb from the fridge and allow to come to room temperature before placing it in the oven to roast. This should take around 45 minutes to 1 hour depending on the size of your leg of lamb.
Next, rub the lamb with garlic and rosemary. Crush 5 cloves of garlic and mix with a large handful of chopped rosemary and 1 tbsp of olive oil in a bowl. Using a sharp knife, make at least twenty small holes or slits in the meat, and rub the garlic, rosemary and oil paste all over the meat, paying extra attention to the incisions.
Heat the oven to 180°C / 350°F /gas 5 and while the oven is warming up, heat the other tablespoon of olive oil in a large frying pan and brown the lamb all over. Meanwhile, scatter the carrot, red onion, leek, potatoes and the rest of the garlic cloves and rosemary in a large roasting tin, pour in the stock, and finally, place the browned lamb on top.
Roast for about 1 hr 30 mins for a 2kg leg of lamb, and 2 hours for a 3kg leg of lamb. Ensure that you turn the lamb halfway through cooking so that both sides of the lamb leg are exposed to the stock. Once the lamb has cooked, take the lamb out of the oven, cover in foil and allow to rest in a warm part of your kitchen for about 30 mins