Sprinkle beef on all sides with black pepper. Wrap beef cubes with enough prosciutto to cover entirely. Be sure that prosciutto doesn't overlap more than once. Place wrapped cubes seam side down on a plate. In a large skillet over medium heat, add oil and butter. When butter has melted add beef cubes, seam-side down to seal, then saute until crisp on all sides, about 8 to 10 minutes for medium rare. Remove beef and let rest 5 minutes before serving. Add shallots to pan and saute 1 minute to soften. Add chicken stock and scrape up brown bits stuck to the bottom of the pan with a wooden spoon. Whisk in mustard and heavy cream and cook 1 minute more to thicken sauce. Remove from heat. Season, to taste, with salt and pepper and garnish with parsley.