Use a fork to poke holes in both sides of a tri-tip. Place the tri-tip in a non-reactive container or vacuum sealed food bag. Add the marinade and cover or seal.
Refrigerate for 8 hours. Turn it over after 4 hours.
Remove the tri-tip from the refrigerator 45 minutes before cooking.
Preheat a cast iron skillet over medium-high heat to about 400°F.
Remove the tri-tip from the marinade, discard the marinade and pat the roast dry. Place a temperature probe into the thickest part of the roast. Once your skillet is hot, add in a small amount of oil, and then sear that roast for about 3-4 minutes on each side. You want to get a nice brown crust on it.
Next, reduce the heat to medium-low. Continue the cooking the tri-tip to your desired level of doneness. I like my tri-tip medium-rare, so I cooked it for about 10-12 minutes total.
Remove the roast from the skillet when the internal temperature is 5°F below your desired temperature. The internal temperature will continue to climb while it is resting. Cover the roast with foil and let it rest 5-8 minutes. Slice across the grain and serve.