Preheat the oven to 170 degrees.
Blitz together the butter and anchovies and spread over the butterflied lamb (leaving the skin side on the outside. Sprinkle over the whole garlic cloves and lay the rosemary along one side then roll tightly and tie. Allow the lamb to come to room temperature before placing in the oven and roasting for 3.5 hours. Rest and keep warm with foil for at least 20 minutes once cooked.
Boil a large pan of salted water and blanch the whole cabbage leaves for one minute then remove and place in iced water or serve hot straight away.
For the salad combine all ingredients, blitz together dressing ingredients and pour over just before serving.
Serve a generous sliced of the rolled lamb on a whole cabbage leaf with a heaped serving of celeriac salad.