Pound the meat slightly between sheets of plastic wrap to a uniform thickness of 1/4-inch. Sprinkle lightly with salt and pepper and top each piece with 2 sage leaves. Cover the sage with the prosciutto slices and skewer into place with toothpicks.
In a saute pan, heat the oil over medium-high heat. Add the meat, prosciutto and sage-side down, and saute until golden on the first side, about 3 minutes. Turn and cook the other side, about 3 minutes longer. Remove to a warmed plate and keep warm.
Pour in the stock and the wine and deglaze the pan over high heat, scraping up any brown bits stuck to the bottom. Boil until the pan juices reduce and are thickened. Remove the pan from the heat and swirl in the butter. Spoon over the saltimbocca and eat at once.
Recipe courtesy of Solo Suppers, Chronicle Books, 2003