In a small bowl, combine crushed garlic, rosemary, lemon zest, one tablespoon oil, salt and pepper. Rub the mixture onto all sides of both tenderloins.
In a large skillet with an oven proof handle, heat 1 tablespoons of oil over medium heat. Add the tenderloins and cook, turning frequently, until meat is seared on all sides, about 8 minutes.
Transfer skillet of pork to the oven and cook for 12 minutes, or until internal temperature is 145°F. (See Note 4)
Remove pork from pan, transfer to a cutting board and lightly tent with foil to keep warm.
Set the skillet over high heat and stir in stock and vinegar, scraping up any cooked bits from bottom of pan. Bring mixture to a boil and cook until reduced by half. Turn the heat off and whisk in the butter, 1 tablespoon at a time, until melted. Stir in the capers and spoon over pork.
Cut the tenderloin into thick 1" slices, spoon caper sauce over the top and serve.
Be sure to purchase a pork tenderloin and NOT a pork loin. There is a difference!
Pork tenderloins are almost always sold two to a package, averaging 1.5 lbs each. Here is a great reference on the two different cuts of pork: The Difference Between Pork Loin and Pork Tenderloins.
You could also use Meyer lemon zest. Meyer lemons are a cross between a lemon and an orange.
Feel free to substitute 1/4 cup of balsamic glaze for the 1/2 cup balsamic vinegar. It's already reduced and thicker.
Don't overcook your pork, keep it nice and moist, not dried out. Internal temp should be between 145°-160°F and allow meat to rest at least 3 minutes.