Rinse the meat under cold water and dab dry carefully with kitchen roll. Finely chop and mix together the rosemary, sage and garlic, and rub onto the lamb with salt and pepper. Place the meat in an ovenproof casserole or roasting pan. Cover with plastic foil to make it airtight and leave to marinate overnight in the fridge.
Roasting the next day:
Pre-heat the oven to 170° C (upper and lower heat). Peel the potatoes and cut into equally sized pieces. Carefully brush oil onto the lamb and put into the oven. Spoon the meat juice over the meat a few times and turn the meat every now and again. After about one hour, mix the wine together with the vinegar and pour over the lamb a spoonful at a time. Increase the temperature to 220° C. As soon as the lamb turns a nut-brown colour, add the potatoes. Stew for another 30 minutes. Turn the meat again so that it is evenly roasted.