Preheat oven to 350 degrees F (175 degrees C).

Saute spinach, butter, onions and garlic over medium heat until soft. Add bread crumbs. Cut tenderloin in half lengthwise. 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. An instant-read thermometer inserted into the center should read 145 degrees F (63 degrees C).