In a kettle combine the corned beef with enough cold water to cover
it by 2 inches, bring the water just to a boil, skimming the froth,
and simmer the corned beet, covered, for 3 hours, or until it is
tender. (Do not let the cooking liquid boil.) Remove the kettle from
the heat and let the corned beef stand in the cooking liquid 20
minutes. In a small bowl stir together the horseradish and the
mustard. Transfer the corned beef to a cutting board and with a sharp
knife remove all but a very thin layer of fat. Cut the corned beef
across the grain into 1/4-inch-thick slices and serve it with the
sauce.

Serves 4 generously.