Saute spinach, butter, onions and garlic over medium heat until soft. Add bread crumbs. Cut tenderloin in half lengthwise. Mix Feta cheese with spinach mix. Stuff with spinach mixture and secure with kitchen twine.
In a bowl, mix ketchup, orange juice and teriyaki sauce. Baste tenderloin with 1/2 of the mixture. Sprinkle pork with cumin. Place in shallow roaster and bake at 350 degrees uncovered for one hour. Baste with reserved marinade and cook one more hour covered. Use meat thermometer to check for doneness. Thermometer should read 160 to 170 degrees F (71 to 77 degrees C).