a smoker box; hickory or mesquite wood chips; a probe thermometer
Combine the pepper, salt, coriander seeds and mustard seeds in a mortar and pestle and grind until coarsely ground. Stir together with the brown sugar, paprika, garlic powder and onion powder. Pat dry the roast with paper towels and evenly coat with the spice rub, making sure to get the sides as well. Put on a rimmed baking sheet, cover and let sit in the refrigerator for at least 30 minutes, ideally overnight.
Heat a grill for indirect cooking to 225 to 250 degrees F. While the grill is heating, prepare a smoker box with hickory or mesquite wood chips. When the wood chips start to smoke, put the box on the back of the grill over direct heat.
Put the tri-trip on the indirect heat side of the grill, leaving the dampers of the grill open. If your grill doesn't have a damper, leave a gap in the lid to let the air in but not let the smoke out. Using a probe thermometer, bring the meat to 130 degrees F, about 1 hour and 15 minutes.
Let the meat rest 10 to 15 minutes, then slice against the grain and enjoy with BBQ sauce alongside.