Preheat oven to 375°F. Position knife at end of loin and push into center as far as it will go, being careful not to cut edge of loin. Pull knife out and rotate 90°. Push knife back into loin, making sure to cut through other cut, but again not through edge of loin. Repeat this step on other end (each end should then have plus sign cut into it). Combine spinach, tomatoes, feta, olives, basil, and oregano in bowl. Mix well. Stuff pork loin by pushing spinach mixture into cuts on each end, using handle of wooden spoon to make sure spinach mixture gets all the way to center. Season outside with salt and pepper. Place on wire rack on cookie sheet. Roast for about 1 hour or until center of pork loin reaches 140°F. Remove from oven. Let rest for about 10 minutes before slicing. Serves 6–8.