Preheat the oven to 500*. In a heavy 10- to 12-inch skillet, heat the olive oil until a light haze forms over it. Add the lamb, 4 or 5 chunks at a time, and brown it over moderately high heat.
As the chunks become richly colored on all sides, transfer them to a heavy 2 1/2- to 3-quart flameproof casserole. Then pour off all but a thin film of fat from the skillet and set the pan aside.
Season the lamb with garlic, rosemary, salt and pepper, and sprinkle it with flour, tossing the meat with a wooden spoon to coat the chunks as evenly as possible with the seasonings and flour.
Place the casserole in the upper third of the oven and brown the lamb uncovered for 8 to 10 minutes, turning it 2 or 3 times.
When no trace of gummy flour remains and the chunks of lamb are lightly crusted, remove the casserole from the oven and reduce the oven heat to 350*.
Pour the vinegar into the waiting skillet and boil it briskly over high heat for a minute or two, scraping in any browned bits clinging to the bottom and sides of the pan.
Add the chicken stock to the vinegar, bring to a boil, and pour the liquid over the lamb in the casserole. Cover the casserole and place it in the middle of the oven.
Braise the lamb for about 1 hour, or until it is tender when pierced with the tip of a sharp knife. (If the braising sauce seems to be cooking away too fast, add a little more stock, a Tblsp. at a time.).
Just before serving, ladle 1 or 2 Tblsp. of the sauce from the casserole in to a small bowl and mix in the chopped anchovies. Stir the anchovy mixture into the casserole and cook for 2 or 3 minutes more. Serve the lamb directly from the casserole or in a deep, heated platter.