A wrapping of pancetta (Italian bacon) keeps roast pork loin moist and imparts delicious flavor and crunch.
Preheat oven to 375 degrees. Season pork with salt and pepper. Heat oil in a large cast-iron skillet over medium heat. Sear pork on all sides until browned, about 10 minutes total. Remove from heat.
Rub pork with chopped rosemary. Wrap with pancetta, overlapping strips slightly. Use toothpicks to secure pancetta, if necessary. Place a rosemary sprig on top; tie pork with kitchen twine, and remove toothpicks. Return pork to skillet. Scatter onions and rosemary sprigs around pork. Roast in oven, basting occasionally with cooking juices, until it reaches an internal temperature of 145 degrees on an instant-read thermometer, 30 to 35 minutes. Remove from oven. Transfer pork and onions to a platter; cover to keep warm.
Make pan sauce: In a small bowl, combine butter and flour. Pour off fat from skillet; place over medium heat. Add stock, scraping bottom of skillet to loosen browned bits. Bring to a boil; reduce liquid slightly, about 2 minutes. Whisk in butter mixture; cook until thickened. Season with salt and pepper. Slice pork, and drizzle with sauce. Serve with onions, polenta triangles and steamed spinach.