1. Rub roast with salt and pepper. Cook in hot oil in a Dutch oven or large cast-iron skillet over medium-high heat 2 to 3 minutes on all sides until browned. Place roast, onion, and next 6 ingredients in a 6-qt. slow cooker.
2. Cover and cook on HIGH 6 to 8 hours or until meat is tender. Remove roast and vegetables; discard vegetables. Shred meat. Pour liquid from slow cooker through a fine wire-mesh strainer into a 4-cup measuring cup, and let stand about 15 minutes. Remove fat from cooking liquid, and discard.
3. Stir together shredded meat, horseradish, next 2 ingredients, and 1 cup reserved cooking liquid; discard remaining liquid. Add salt and pepper to taste. Serve on rolls with Pickled Peppers.
Note: Keep beef mixture warm in slow cooker on WARM up to 2 hours. We tested with Pepperidge Farm Stone Baked French Artisan Rolls.