Remove any excess fat from the roast, leaving a thin layer. In a small bowl, combine dry mustard, sugar, and Dijon mustard. Brush mixture over the fat and cut surfaces of the roast. Refrigerate for at least 2 hours or up to overnight.
Preheat oven to 450 degrees F.
Set the roast, rib-side down, in a heavy, shallow roasting pan. (The ribs act as a natural rack.) Using a paring knife, score the fat. Season with salt and pepper. Roast for 15 minutes. Reduce heat to 350 degrees F and continue to roast, basting every 15 minutes, until it reaches a temperature of 125 degrees F for medium rare on an instant-read thermometer. Remove roast to a platter. Let rest for 15 minutes before carving.
Pour off all but 2 tablespoons of fat from the roasting pan. Set pan on stove over medium heat. Simmer until juices begin to darken, 1 to 2 minutes. Whisk in flour and cook, scraping up caramelized bits, until flour is deep golden brown, about 3 minutes. Add stock and bring to a boil, stirring until thickened. It should very lightly coat the back of a spoon. Season with salt and pepper. Strain gravy and serve with potatoes, Brussels sprouts, and Horseradish Sauce.
3 pounds Idaho potatoes, peeled and halved
1/2 cup drippings
Preheat oven to 400 degrees F.
Place potatoes and salt in a large saucepan. Add enough water to cover. Bring to a boil and cook for 5 minutes. Remove from heat and drain. Score potatoes all over with the tines of a fork.
Combine cooked potatoes and drippings in a roasting pan large enough to allow the potatoes to roast in a single layer. Transfer to oven and roast until crispy and golden brown, about 1 hour.
1 heaping tablespoon freshly grated horseradish or 3 tablespoons bottled
Coarse salt and freshly ground pepper
1/2 lemon, juiced
1 cup heavy cream, whipped to soft peaks
Stir horseradish, salt, pepper, and lemon juice into whipped cream.
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