Marinate overnight: combine all marinade ingredients in a small bowl and whisk together. Score the pork tenderloins 4-5 times on each side with a sharp knife - shallow cuts, about ¼ inch deep. This allows the marinade to penetrate. Place tenderloins in a zip-lock bag or covered dish, pour marinade over, massage in thoroughly. Refrigerate 8 hours or overnight. Minimum 4 hours if pressed for time.
Bring to room temperature: remove pork from refrigerator 30 minutes before cooking. Pat completely dry with paper towels - this is essential for the crust. Mix the sazon rub ingredients together and press all over both tenderloins.
Sear: preheat oven to 425F. Heat olive oil in a large oven-safe skillet over high heat until just smoking. Sear tenderloins 2 minutes per side, all four sides, until deeply browned. Do not move them during searing. Total sear time approximately 8 minutes.
Roast: add sliced onion and whole garlic cloves to the pan around the pork. Pour ½ cup bone broth into the bottom of the pan - not over the pork. Transfer to oven. Roast 20-25 minutes, basting with remaining bone broth every 8 minutes, until internal temperature reads 145F on an instant-read thermometer.
Rest: transfer tenderloins to a cutting board and tent loosely with foil. Rest 10 minutes minimum - do not skip this. The internal temperature will rise to 150-155F during resting. The juices redistribute during this time. Cutting before resting loses all of them.
Make the pan sauce: place the skillet over medium heat on the stovetop. Add remaining bone broth and scrape up all the browned bits from the bottom. Simmer 2-3 minutes until slightly reduced. Taste and adjust salt. Pour over sliced pork at the table.
Slice and serve: cut tenderloins into ½-inch medallions against the grain. Arrange on a platter with the roasted onions and garlic. Spoon pan sauce over the top. Garnish with fresh cilantro and a few slices of naranja agria.