Rinse beef, pat dry, and sprinkle generously all over with fresh-ground pepper. Melt butter in a 10- to 12-inch nonstick frying pan over high heat. When hot, add beef and brown well on all sides, 6 to 8 minutes total.
Meanwhile, cut carrots into sticks about 3/8 inch thick and 2 inches long. In a 4 1/2-quart or larger electric slow-cooker, combine carrots, onion, celery, garlic, thyme, peppercorns, and bay leaf. Set beef on vegetables; add drippings. In a small bowl, mix wine and tomato paste; pour over meat and vegetables.
Cover and cook until beef is very tender when pierced, 8 to 9 hours on low, 5 to 6 hours on high. If possible, turn meat over halfway through cooking.
With 2 slotted spoons, transfer meat to a platter; keep warm. Skim and discard any fat from cooking liquid. Turn cooker to high. In a small bowl, blend cornstarch with 1 1/2 tablespoons water; pour into cooker and stir often until sauce is bubbling, 10 to 15 minutes.
With a slotted spoon, lift vegetables from cooker and arrange beside meat. Spoon horseradish smashed potatoes onto platter; sprinkle with parsley. Garnish platter with watercress. Spoon sauce over meat. Slice meat and serve with vegetables, potatoes, and sauce, adding salt to taste.
Nutritional analysis per serving without potatoes.