Preheat oven to 425°F/225°C.

Add the sage, garlic powder, salt and pepper to a bowl and mix well.

Drizzle the tenderloin with oil and rub all over. Sprinkle the spice mix all over the pork and rub well with hands.

Preheat a large cast iron skillet or oven-proof skillet on the stove over medium-high heat for 2 minutes. Add the pork and seer on each side for 3 minutes each.

Transfer the pan to the oven and roast for 15-17 minutes until a meat thermometer measures 140°F/60°C. Remove from the oven and allow the pork to rest on a cutting board 10 minutes before slicing. The pork will continue to cook when it comes out of the oven and 145°F/63°C is the final temperature it needs to reach using a meat thermometer.