Return the skillet over medium heat with 1 more turn of the pan of EVOO, about 1 tablespoon, and add the pancetta. Cook until crispy and golden brown, about 5 minutes, then add the onion and chopped rosemary to the pan and cook until the onions are tender, about 5 minutes. Add the balsamic vinegar to the pan and cook until reduced, about 2 minutes, then season the mixture with salt and pepper. Top the seared pork loin with the pancetta-onion mixture and roast until done to your liking, about 1 hour (internal temperature should be 140ºF for medium).
When the pork is ready, transfer it to a cutting board and tent with foil to rest. Place a medium skillet over medium heat with the butter. When the butter has melted, sprinkle the flour over top and cook for about 1 minute. Add the white wine to the pan and whisk to incorporate. Pour off the pan juices from the roasting pan into the skillet, removing the rosemary sprigs (the rosemary can be discarded or saved for garnish on a serving platter). Add the Worcestershire and stock the pan, and bring up to a bubble. Simmer the gravy until thickened, about 2 minutes, and season with salt and pepper.