Preheat oven to 400°. Line a baking try with foil, spray with cooking spray and place in oven. Remove fat from pork and butterfly the meat. Cut it nearly in half lengthwise. Open the pork and lay flat like a book.
Pound it flat with the palm of the hand or the bottom of a skillet. Chop rosemary if using fresh. Rub pork with olive oil and sprinkle rosemary on both sides. Remove baking tray from the oven and place pork on hot tray.
Return to oven and roast 15 minutes. Remove and let sit 5 minutes, then carve.