With a tenderizing mallet, proceed to tenderize pork loin thoroughly - making effort to really work the tougher portion.
Finely dice garlic and rosemary, then put into small bowl. Add in seasonings and olive oil, mixing to make into paste.
Press paste firmly onto all sides of the tenderloin.
In large, oven - proof skillet, add 1 Tbsp olive oil over med- high heat. Add tenderloin, and brown all sides (will take roughly 10min) There will be browned bits stuck to bottom of pan, do not worry about that- its essential to the sauce.
Transfer to oven and roast for 12 min (or until temp is 165°- keeping in mind though that after removed from oven the meat will continue to cook.) ****I usually will remove any meats im cooking based on temp roughly 4-5° before my desired temp).
Remove pork from pan and set aside.
Set pan over high heat and add balsamic vinegar and beef stock- using a wooden spoon to scrape up the baked on browned bits on bottom of pan. Bring this to a boil, and cook until reduced by half, making sure to stir frequently. It will only take about 5 minute.
Turn off heat, and whisk in butter.
Serve spooned over the sliced pork tenderloin.
Dish is excellent with roasted baby potatoes, steamed green beans/ asparagus and/ or salad.