Remove excess fat from roast, leaving just a thin layer.
In small bowl, combine dry mustard, sugar, and Dijon mustard. Brush mixture over fat and cut surfaces of roast. Refrigerate 2 hours to overnight.
Set roast, rib-side down, in heavy, shallow roasting pan in 450 F oven. (Ribs act as a natural rack.) With a paring knife, score the fa and season with salt and pepper.
Roast 15 minutes. Reduce heat to 350 F and continue to roast (basting every 15 minutes) until temperature reaches 125 F (medium rare). Remove roast to a platter. Let rest for 15 minutes before carving.
GRAVY: Keep 2 tablespoons of fat from roasting pan. Set it on stove over medium heat and simmer until juices begin to darken, 1/2 minutes. Whisk in flour and cook, scraping up caramelized bits, until flour is deep golden brown, 3 minutes. Add stock and bring to boil, stirring until thickened. Season with salt and pepper.