For the mutton, season the leg all over with salt and freshly ground black pepper. Rub the inside of an ovenproof casserole dish with the butter. Place the mutton leg into the dish and scatter the sliced onions on top.
Wrap the rosemary, bay leaves and peppercorns in a small piece of muslin cloth and tie with a piece of kitchen string into a little parcel. Place into the casserole dish.
Pour in white wine and lamb stock. Butter a cartouche of greaseproof paper large enough to cover the casserole and place over the mutton, tucking in the edges.
Place into the oven for 3 hours, or until the mutton is meltingly tender.
Meanwhile, for the caper sauce, simmer the chicken stock in a small pan until reduced in volume by half. Stir in the cream and the capers and simmer for 2-3 minutes, then season to taste with salt and freshly ground black pepper. Set aside.
Remove the mutton from the oven. Spoon the cooked onions onto a large serving plate with a slotted spoon. Thickly slice the mutton and lay the slices over the onions. Spoon over the warm caper sauce and serve immediately.