Preheat the oven to 375°F (190°C).
Pat the 2 lb tri-tip dry and trim excess fat if needed.
Rub the tri-tip all over with 1 tbsp olive oil to coat evenly.
Sprinkle 3 tbsp steak rub generously over the meat and massage it into all sides.
Heat a cast-iron skillet over medium-high until very hot, then sear the tri-tip 2 minutes per side to form a deep brown crust.
Insert a meat thermometer into the thickest part of the roast, then transfer the skillet (or move the tri-tip to an oven-safe dish) to the preheated oven.
Cook in the oven until the internal temperature reaches 125°F for medium-rare, or to your desired doneness (monitor with the thermometer).
Remove the tri-tip from the oven and transfer to a cutting board; let rest 5–10 minutes so juices redistribute.
Slice thinly against the grain and serve.