1.Rinse beef under cold water; pat dry with absorbent paper. Place beef, onion, carrot, celery, peppercorns, vinegar and sugar in a 4.5-litre (18-cup) slow cooker.
2.Add enough of the water to barely cover beef. Cook, covered on low for 8 hours.
3.Remove beef from cooker; discard liquid and vegetables. Cover beef to keep warm.
4.Make horseradish sauce. Heat milk until hot. Melt butter in a medium saucepan, add flour; cook, stirring, 1 minute. Gradually add milk, stirring, until sauce boils and thickens. Stir in horseradish cream and parsley. Season with salt and pepper.
5.Serve beef with sauce.
The horseradish sauce is best made just before serving. However, if you make it earlier, cover the surface of the sauce with plastic wrap to stop a skin from forming. Reheat gently in a saucepan, stirring, until heated through.