Prepare the tri tip by trimming any excess fat. Generously season the tri tip with the beef rub on all sides. Do this ahead of time at least an hours before cooking, up to two days. Keep covered while refrigerated.
Preheat Traeger Grill to 425F with the lid closed and place a cast-iron pan in the hottest area of the grill.
When the Traeger has reached 425F, lightly drizzle the cast iron with vegetable oil. Lay the tri tip in the cast iron and cook for 5 minutes with the lid closed. After 5 minutes, flip the tri tip and cook for another 5 minutes.
After the tri tip has cooked in the cast iron, lower the temperature on the grill to 325F. Move the tri tip to the opposite side of the grill without the cast iron pan.
Cook the tri tip at 325F with the lid closed for 20-25 minutes or until the internal temperature reaches 130F.
After cooking, remove the tri tip to a large cutting board and cover with aluminum foil. Allow the meat to rest for 10-15 minutes before cutting.
Slice the tri tip in ¼" slices going against the grain. Enjoy while its hot and serve with a good BBQ sauce if desired.