Prepare the tenderloin: Use paper towels to grab the silver skin and pull it away from the tenderloin. With a boning knife or small knife, trim off any remaining big pieces of fat. Slice the tenderloin first into medallions, about 1-inch thick. Then slice the medallions into bite-sized cubes, about 1-inch.
Make the marinade: In a large bowl or zip lock bag, combine the white wine vinegar, olive oil, garlic cloves, mustard, oregano, rosemary, paprika, and black pepper.
Marinate the pork: Add the pork and toss well to combine. Cover or seal and refrigerate for at least two hours. If you have the time, allow the meat to marinate overnight. (During that time the color of the meat will get lighter, because of the vinegar.)
Get ready: Remove the bowl from the refrigerator about 30 minutes before you are ready to grill. If you are using wooden skewers, soak them in water for 30-40 minutes so they do not burn on the grill. (If you are using metal skewers, skip that step.)
Skewer: Slice the onion into quarters and pull apart the layers. Thread the marinated pork through the skewers, alternating with red onion pieces. Depending on the size of the skewers you are using, the quantity of meat will be enough for 4 large or 6 smaller skewers.
Grill: Season the meat with the sea salt and brush everything with the remaining marinade. Grill for about 4 minutes per side, turning to ensure a nice color on all sides. When the meat is done it should have solid grill marks and an internal temperature of 145° F.
Rest: Remove the skewers from the grill and place them on a serving platter. Cover them loosely with foil and allow them to rest for 4-5 minutes.
Serve: Slice the lemon in half and squeeze on some fresh lemon juice. Serve.
Pork tenderloin gets dry and tough when it’s overcooked. Remember: you only need to cook pork to 145°F and it will stay juicy.
You want to grill the meat quickly over relatively high heat. Make sure your grill is nice and hot to get a good sear. This will also prevent sticking!