Cut meat into 4 serving-size pieces. Spread both sides of meat with the mustard; sprinkle with peppercorns.
In a large skillet, heat oil over medium-high heat. Cook meat 3 to 4 minutes per side or until desired doneness. Remove from skillet; keep meat warm. Carefully add broth and vinegar to skillet, stirring to scrape up browned bits. Add sugar and prunes. Boil gently 3 to 5 minutes or until liquid is reduced by half. Whisk in butter to melt. Pour sauce over meat.