Make the rub: using a spice grinder or mortar/pestle, crush the black peppercorns and mustard seeds.
Transfer to a small bowl with the remaining rub ingredients.
Trim excess fat from roast; brush the meat with Worcestershire sauce and coat evenly with the rub.
Cover with plastic wrap and refrigerate 8-12 hours.
Let roast stand at room temperature for 30-40 minutes before grilling.
Grill the roast over indirect medium heat, keeping the grill’s temerpature between 350° and 375°, until the internal temperature reaches 135° for medium rare, about 2 hours.
Remove roast from the grill, loosely cover with foil, and let rest for 20-30 minutes.
The internal temperature will rise 5-10° during this time.