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Preheat the oven to 275 degrees.
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Place the corned beef in a colander set in the sink and rinse well under cold running water.
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Place the beef in a large French oven with a tight fitting lid. Add the water, bay leaves, peppercorns, allspice, cloves, carrots and onions. Bring to a boil, reduce to a simmer, and skim off any scum that rises to the surface. Cover and transfer pan to the oven, and braise until very tender, about 4 hours.
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While the corned beef is braising, prepare the horseradish sauce. In a small bowl, mix together the mayonnaise, sour cream, horseradish and zest. Season with salt and pepper to taste. Refrigerate the horseradish sauce for at least 30 minutes before serving.
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Transfer the corned beef to a cutting board and cover tightly with foil to keep warm.
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Add the cabbage and potatoes to the cooking liquid and bring to a boil. Lower the heat and simmer until the vegetables are tender, about 20 minutes.
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Using a slotted spoon, transfer the cabbage to a large platter. Slice the corned beef across the grain of the meat into thin slices. Lay the slices over the cabbage and surround it with the potatoes, carrots and onions.
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Ladle some of the hot cooking liquid over the corned beef and serve with the horseradish sauce.
