Heat an ovenproof 6-quart Dutch oven over medium heat. Add oil; swirl to coat. Add roast to pan. Cook 10 minutes, turning to brown on all sides. Remove from heat. Add onion, carrot, garlic, and rosemary to pan. Sprinkle roast and vegetables with salt and pepper. Add 1/2 cup water to pan. Cover and bake at 325° for 3 to 3 1/2 hours or until meat is fork-tender, turning roast halfway through cooking time.
While roast bakes, combine sour cream and horseradish in a small bowl.
Skim fat from drippings. Break roast into large chunks with 2 forks. Serve roast with vegetables, pan drippings, and horseradish cream. Garnish with parsley, if desired.
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