Step 1

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.

Step 2

Heat oven to 450 degrees. 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 and continue to roast, basting every 15 minutes, until it reaches a temperature of 125 degrees for medium rare on an instant-read thermometer. Remove roast to a platter. Let rest for 15 minutes before carving.

Step 3

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.