1. To make the marinade; crush the garlic along with the dry spices until the mix becomes almost a paste. Add the onion chopped or sliced into thin rings and the Seville orange or grapefruit juice.
2. Trim the pork tenderloin of any excess fat and silver skin.
3. Mix all of the marinade ingredients together and pour half of a mixture into a large resealable bag. Add the pork tenderloin to the bag removing as much air as possible before sealing. Place the bag in the refrigerator and marinade for at least an hour, up to 24 hours. Put the remaining marinade in a covered container and refrigerate until ready to use.
4. Remove the tenderloins from the marinade about 30 minutes before you are ready to grill and allow to sit at room temperature while preparing the grill. Remove the reserved marinade from the refrigerator.
5. Heat the grill to medium-high heat.
6. Once the grill is heated, place the tenderloins on the grill and grill the first side for around 5 minutes. After 5 minutes, roll the tenderloin onto the the next side and grill for another 5 minutes. Cook for 12 to 15 minutes, turning every 5 minutes, until the tenderloin reaches an internal temperature of 60 degrees Celsius (140 F).
7. Remove the tenderloin from the grill and allow it to rest on a large piece of heavy-duty aluminum foil folded at the edges to create a basket, and pour on the reserved marinade. Wrap tightly and let stand for 10 minutes. Cut into slices, and serve.