How to Make It
One to 2 days ahead, put the prunes in a nonreactive bowl and cover with the Madeira; set aside at room temperature to plump.
Preheat the oven to 450°. In a large, heavy flameproof roasting pan set over 2 burners, heat the oil over moderately high heat. Season the beef with salt and pepper. When the oil is hot but not smoking, add the meat to the pan and brown it well on all sides.
Transfer the roasting pan to the oven and roast the meat for about 20 minutes, or until an instant-read thermometer inserted in the thickest part registers 125° for medium rare. Set the roast on a large warmed platter, cover loosely with foil and keep warm.
Discard the fat in the roasting pan. Set the pan over 2 burners, add the shallots and red wine and simmer over moderately low heat until the wine is reduced by half, about 8 minutes. Increase the heat to moderately high, add 1 1/2 cups of the stock and boil until reduced by half, about 5 minutes. Add the prune soaking liquid and boil until reduced to 1 1/2 cups, about 7 minutes. Pour the sauce into a nonreactive medium saucepan. Add the soaked prunes and the remaining 1/2 cup stock and simmer over low heat, stirring occasionally; the sauce will thicken slightly.
Cut the beef into 16 slices. Pour any meat juices into the sauce and stir in the butter, 1 tablespoon at a time. Season the sauce with salt and pepper and serve alongside the beef.