Trim fat on roast to 1⁄4 inch thick. Place roast on work surface, meat side down with ribs curved up. Cut 1 1⁄2-inch-long slits (about 3⁄4 inch deep) between rib bones on bony side of roast. Stand rack with ribs pointed up and slits facing away from you. Curve rack so that ends meet and form a crown. Tie roast with kitchen twine at bottom, in middle, and around bones to secure. (You can ask a butcher to do this step.) Wrap top of each rib bone, individually, with aluminum foil to prevent burning.

Stir together mustard, garlic, thyme, rosemary, salt, oil, pepper, and sugar in a small bowl. Rub mixture on roast, working it into slits and avoiding frenched bones. Place roast on a rack set in a foil-lined rimmed baking sheet. Chill, uncovered, 8 hours or up to overnight.

Remove roast from refrigerator, and let stand at room temperature 1 hour. Preheat oven to 450°F with oven rack in lower third of oven. Bake roast in preheated oven 15 minutes. Reduce oven temperature to 325°F, and continue baking until a thermometer inserted into thickest part of rib registers 135°F for medium, 1 hour and 50 minutes to 2 hours and 10 minutes. Remove roast from oven; tent with foil. Let rest 30 minutes (internal temperature will rise to 145°F as the meat rests). Remove kitchen twine and foil caps before slicing between bones and serving.