1 Preheat the oven to 170°C (150°C fan) 325°F gas 3.
2 Pierce the meat all over with a sharp knife. Place the rosemary in the slits. If necessary tie with kitchen twine.
3 Mix together the garlic and 4 tablespoons oil. Rub the mixture into the meat and season with salt and pepper. Place in a baking dish and roast for 3 hours. If necessary add a little water and turn the meat occasionally.
4 Fry the mushrooms in the remaining oil, remove from the heat and add the balsamic vinegar. Season with salt and pepper and arrange around the meat before serving.