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.
Servings Makes 6 to 8 servings
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.