Cut pork lengthwise, cutting to, but not through, other side. Open halves, laying pork flat. Place pork halves between 2 sheets of heavy-duty plastic wrap; pound to an even thickness using a meat mallet or small heavy skillet. Spread pesto evenly down length of tenderloin; top with spinach and feta. Close tenderloin; secure at intervals with wooden picks (about 12). Place pork in a 13 x 9-inch baking pan coated with cooking spray. Coat pork with cooking spray; sprinkle evenly with pepper. Pour tomatoes around pork in pan.
Bake at 500° for 22 minutes or until a thermometer registers 160° (slightly pink). Let stand 5 minutes. Discard wooden picks; cut pork into 8 slices. Serve roasted tomatoes over pork.