Trim fat from roast. Coat a Dutch oven with cooking spray; place over medium-high heat until hot. Add meat, and cook until browned on all sides; sprinkle with garlic, salt, and pepper. Pour broth over meat; bring to a boil. Cover, reduce heat, and simmer 1 hour and 45 minutes. Add carrot and next 3 ingredients; bring to a boil. Cover, reduce heat, and simmer 45 minutes or until meat and vegetables are tender.
Combine sour cream and horseradish; set aside.
Transfer meat to a serving platter. Arrange vegetables around roast. Let meat stand 10 minutes before slicing. Serve with sour cream mixture. Garnish with rosemary sprigs, if desired.