Preheat the oven to 160°C fan and get the lamb to room temperature
Mix the butter together with the garlic, the 2 sprigs of finely chopped rosemary, mustard, lemon zest, anchovies if using, and plenty of salt and pepper
Prick a few little holes into the lamb and then rub the butter all over
Chuck the onions into a 24x32cm roasting tin and add the remaining two sprigs of rosemary
Top with the lamb and pour the stock around the edge
Cover with a huge piece of wet baking paper and plenty of foil
Roast for 3½ - 4 hours or until the meat pulls apart with two forks, basting the meat with the juices
Cover again and leave to rest for 20 minutes before pulling apart to serve
