In a large skillet, brown roast on all sides in oil over medium-high heat; drain. Place carrots and potatoes in a 5-qt. slow cooker. Top with meat and onions. Combine the water, horseradish sauce, vinegar, Worcestershire sauce, garlic and celery salt; pour over meat. Cover and cook on low until meat and vegetables are tender, about 8 hours., Combine cornstarch and cold water until smooth; stir into slow cooker. Cover and cook on high until gravy is thickened, about 30 minutes.