DO-AHEAD TIP: Marinate pork for at least 1 hour or up to 24 hours (see recipe). If using bamboo skewers, soak them in water for 30 minutes before use, to prevent burning.
In a large zipper-topped plastic bag, combine first 6 ingredients (vinegar through pepper); seal bag and shake to combine well.
Open bag and add pork; seal bag and refrigerate for at least 1 hour or up to 24 hours. Remove pork from marinade and place the marinade, minus the garlic, in a small saucepan; bring to a boil and cook until slightly thickened and reduced by half, 5 to 7 minutes; remove from heat and set aside.
Preheat indoor or outdoor grill to HIGH. Alternately thread pork cubes, apple chunks and red onion chunks onto skewers; sprinkle with salt and pepper.
Grill kabobs for 4 minutes then turn them a quarter of the way around and grill for another 4 minutes. Grill remaining two sides for 4 minutes per side, for a total grilling time of 16 minutes.
Drizzle with reserved marinade.