Cover a sheet pan with a layer of plastic wrap.

In a small bowl combine the spice rub ingredients with 2 tablespoons of oil. Mix well to create a paste.

Using your hands, rub the spice mixture all over the roast, massaging it into the meat. Pull the plastic wrap over the roast and wrap it tightly. You can keep the wrapped roast on the sheet pan or you can place it inside a large resealable bag.

Marinate for 30 minutes at room temperature or up to 24 hours in the refrigerator.

Remove the roast from the fridge 30-45 minutes before cooking it, so the meat has time to come to room temperature.

Preheat the oven to 400ºF (204ºC).

In an oven-safe skillet or cast iron skillet, heat the remaining 2 tablespoons of oil over medium-high heat. When the oil is hot, add the tri tip to the skillet (fat side down), and gently press the roast down so it makes good contact with the skillet. Sear for about 3 minutes or until a nice, golden brown crust has developed.

Using kitchen tongs, flip the roast and sear the other side for about 2 minutes then, transfer the skillet to the pre-heated oven.

Roast for about 20 to 25 minutes (around 10 to 15 minutes per pound), or or until an instant read thermometer inserted into the thickest part of the meat reads 120°F to 125º F (49ºC to 51°C) for medium-rare or 130º to 135º F (54ºC to 57ºC) for medium. The internal temperature of the meat will rise while the roast is resting, due to carryover cooking.

Remove the skillet from the oven. If using butter, slather the butter on top of the roast with a butter knife, and tent the tri-tip with aluminum foil. Allow the meat to rest for 10 minutes.

Transfer the meat to a cutting board and slice it thinly, against the grain. Serve drizzled with any pan juices and a sprinkle of finishing salt (optional).