To cook the pork tenderloin in the oven follow these instructions: Heat a cast iron skillet over medium-high heat on the cooktop. Place the pork tenderloin into the skillet and sear,3 minutes per side, for a total of 6 minutes. Place the pork onto a rimmed sheet pan, place into the oven and cook for 14-20 minutes at 425 degrees, or until pork reaches 140 degrees.