Preheat the oven to 400°F.
Trim any excess fat or silver skin from the pork tenderloin.
Use a small sharp knife to cut small slits all over the pork. Insert the garlic cloves directly into the openings. If any cloves are especially large, halve them first so they fit more easily. (Note: If you prefer a softer, sweeter garlic flavor, roast the garlic first. See How to roast garlic section in Recipe Notes for steps.)
In a bowl, whisk together the balsamic vinegar, olive oil, parsley, rosemary, thyme, sage, sea salt, and black pepper.
Place the pork in a shallow dish and pour the marinade over the top. Coat the meat well and marinate for at least 30 minutes, or up to 8 hours in the refrigerator.
Heat a cast iron or other oven-safe skillet over medium-high heat with a little olive oil.
Remove the pork from the marinade and sear it in the skillet for 1 to 2 minutes per side, until browned all over and nicely caramelized.
Pour any remaining marinade over the pork before placing the skillet in the oven.
Roast for about 15 to 20 minutes, or until the internal temperature reaches 145°F at the thickest part.
Remove from the oven and let the pork rest for 5 to 10 minutes before slicing.