You're going to double butterfly the tenderloin so it can be flattened, stuffed and rolled. To do that, you'll make two long horizontal cuts, one on each side, dividing the tenderloin in thirds without cutting all the way through. Place the tenderloin on a cutting board. Holding the knife blade flat, so it's parallel to the board, make a lengthwise cut into the side of the tenderloin one-third of the way down from the top, stopping short of the opposite edge so that the flap remains attached. Rotate the tenderloin 180 degrees. Still holding the knife parallel to the cutting board, make a lengthwise cut into the side opposite the original cut, two-thirds of the way down from the top of the tenderloin and taking care not to cut all the way through. Open up the two cuts so you have a large rectangle of meat. Use the heel of your hand to gently flatten the meat to about 1/2-inch thick.