Place meat in a casserole with onions, carrots and bay leaves, and cover with water. Season with salt (no salt if salt beef is used) and freshly ground black pepper, and simmer gently for 3 to 4 hours, or until meat is tender. Cool in stock. Slice meat thinly and reheat in stock just before serving. Serve with Horseradish Sauce.
Melt butter in the top of a double saucepan; add flour and cook, stirring constantly, until smooth. Add hot milk, stirring constantly until mixture comes to a boil. Drain horseradish, add it to sauce with lemon juice, and season to taste with salt and freshly ground black pepper.