How to Make It
Cook peppercorns and coriander seeds in a small skillet over medium, stirring occasionally, until fragrant, about 4 minutes. Remove from heat; cool 10 minutes. Coarsely crush with a mortar and pestle, or transfer to a ziplock plastic bag and crush with the bottom of a heavy skillet.
Stir together mustard, salt, and crushed spices in a small bowl. Rub mixture evenly over roast. Place roast, fat cap up, on a wire rack set inside a rimmed baking sheet lined with aluminum foil. Chill, uncovered, 8 hours or overnight.
Remove roast from refrigerator; let stand at room temperature 1 hour. Preheat oven to 300°F. Cook roast in preheated oven until a thermometer inserted in thickest portion registers 120°F for medium-rare, about 2 hours and 30 minutes (or 130°F for medium), rotating pan after 1 hour and 30 minutes. Remove from oven; discard drippings from baking sheet.
Increase oven temperature to 500°F. Let roast rest while oven preheats, at least 30 minutes. Return roast to oven; cook at 500°F until meat is well browned and a crust has formed, 5 to 8 minutes. Slice and serve immediately.