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1
Prepare short ribs:
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2
Pat beef dry. Heat olive oil in a wide (12 inches in diameter), 5-quart heavy oven-safe skillet over moderately high heat until hot but not smoking. Then brown beef on all sides, leaving space between ribs for turning with tongs, about 8 minutes. Transfer to a plate and sprinkle with 1 teaspoon kosher salt and 1/2 teaspoon pepper. Do not discard oil and leavings.
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3
Put rack in middle position in oven and preheat to 325u00b0F.
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4
Make sauce:
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5
In same skillet, over moderately high heat, re-heat the oil and leavings. Add fennel, thyme and garlic and saute. Add onions, carrots, and celery to skillet. Over moderately high heat, saute until onions start to soften, about 15 minutes. Stir in diced tomatoes, mustard, and parsley. Season with remaining salt and pepper and simmer on high heat for 1minute. Add port wine to simmering mixture (glazing pan) and loosen all bits and leavings. Simmer until alcohol is reduced. Add chicken broth and bay leaf; simmer until sauce appears thick and rich.
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6
Transfer short ribs and all juices to the mixture in the skillet, placing them bone side up. Ribs should be spaced evenly without touching. Cover and place in heated oven and braise until meat is tender and falling off bone, about 2 hours. Let cool slightly.
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7
When ribs are cool enough to handle, remove bay leaf and any excess fat. Transfer beef, reserving liquid, to bowl and cover with foil to keep warm.
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8
Make sauce:
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9
Put some of the solids aside, before straining, to use as a garnish. Strain cooking liquid through fine-mesh sieve and into medium saucepan, pressing on solids. Discard solids and skim fat. Whisk in 2 tablespoons flour and bring to a boil. Continue cooking until reduced to about 2 1/2 cups, about 5 minutes.
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10
To serve, place garnish onto plate, top with mash potatoes, lay one rib against the potato bed and spoon braising liquid over meat and finish with horseradish.
