Preheat oven to 425°F; melt butter in a large skillet. Season beef with salt and pepper and add to skillet; cook over medium-high heat to sear well on all sides. Remove from skillet and transfer to a shallow baking dish. Cook for about 30 to 35 minutes or until beef reaches 135°F on a meat thermometer for medium rare; let stand for 10 minutes before slicing. Meanwhile, add stock, prune juice and port to skillet and cook over high heat for 20 to 25 minutes or until mixture has reduced to a fairly thick glaze. Sprinkle with chopped fresh rosemary and serve with glaze.