For the marinade: In a small pot heat the oil, garlic, and cumin. Let the mixure simmer for 2-3 minutes then stir in all of the remaining ingredients and bring to a boil. Remove marinade from the heat and stir in the cilantro after it cools. Pour marinade over pork either in a covered container or in a large Ziploc bag and marinate in the fridge for at least 24 hours.
For the pork: After at least 24 hours, remove the pork from marinade, reserving marinade for later use. Preheat the grill to medium heat. Sear the tenderloin on all sides and continue to grill over medium heat for about 20-30 minutes rotating meat when needed until cooked thoroughly. The cooking time will vary depending how thick your tenderloin is. You want it to be slightly firm not gushy when you touch it with the grilling tongs. You want it to be cooked but not dry. Let pork rest for a couple of minutes before slicing so that the juices can settle back in. Slice pork into 1/2 inch medallions.
For the onions: Slice the onions into thick slices and brush with a little bit of oil. Grill onions when the pork is almost done so that they finish at the same time. Grill for 2-3 minutes per side or until soft.
For the sauce: Take remaining marinade and bring once again to a boil in a small pot. Reduce the heat and let it simmer for 3-4 minutes.
To serve: Pour sauce over sliced pork with grilled onions on the side. For a pretty presentation slice limes and oranges and place them on the plate next to the tenderloin and onions. Serve with a simple salad of mixed greens, cherry tomatoes, and cucumbers tossed in olive oil and red wine vinegar.