In a small bowl, mix together the marinade ingredients.
Butterfly the pork tenderloin by cutting it length wise down the middle until you reach about an inch thick. Put the pork between parchment or plastic wrap and pound until it's about an inch thick.
Using either a large zip lock bag or large tupperware container with a lid, combine the marinade with the pork tenderloin and refrigerate for at least an hour.
Heat up your BBQ to a medium high heat. Once hot, add the marinated tenderloin to the direct heat grill. Tossing once or twice, about 7 minutes a side, cook until you get an internal temperature of 160 F. Remove from the grill and allow to rest for 10 minutes.
Meanwhile, heat a pan on medium heat. Combine the Red Rub, stock, rosemary and bring to a gentle simmer for about 5 minutes. Add the balsamic vinegar and capers to the pan and continue to simmer gently just until the sauce begins to take on a syrup consistency, few more minutes. Off heat, stir in the butter.
Slices the pork tenderloin against the grain, or as you see in picture above - top off the pork tenderloin with the hot pan sauce or put in a bowl on the table for individual servings.