Season the tri-tip. A few hours before you plan to cook, season the tri-tip all over with kosher salt and pepper. Lay the meat on a plate and leave it uncovered in the fridge for up to 8 hours.
Mix the rub. Combine all the rub ingredients in a small bowl and add the olive oil. Stir well until it resembles a paste.
Rub the tri-tip. Massage all the rub into the tri-tip, really taking care to get in all the nooks and crannies. Return the tri-tip to the fridge for 2-4 hours.
Prep the grill. Before using your grill, make sure to clean the grates of any food and debris. Lightly grease the grates with nonstick spray. Preheat an outdoor grill where one side is medium-high heat and the other side is low heat. Close the lid and wait until the temperature reaches 450 F.
Grill the tri-tip. Place the tri-tip on the hot side of the grill and grill 4-5 minutes per side, until browned with grill marks. Transfer to the cool side of the grill (lower the temp on the hot side) until the internal temperature reaches 130 F, about another 20-30 minutes. Remove from the grill, lightly tent with foil, and let the grilled tri-tip rest for 15 minutes before slicing.
Slice against the grain: Because the tri-tip is where several muscles converge, you have muscle fibers running different directions. I find that cutting tri-tip on the diagonal with one point up and two points down, like an isosceles triangle, works best.