Generously salt and pepper the pork roast and cover with chopped rosemary.

Heat a medium skillet over medium high heat and add olive oil. Add pork roast to the skillet seasoned side-down and sear on all large sides until golden brown, about 5 minutes per side. Transfer pork roast to a medium-sized or larger slow cooker.

Reduce stove top heat to medium low and add onions to the skillet. Cook onions until softened and caramelized somewhat in the drippings from the roast. Transfer onions to the slow cooker.

Add the stock, balsamic vinegar, garlic and rosemary sprig to the slow cooker.

Cook on low for about 6 hours or high for about 4 hours, until roast is fork tender.

Remove roast from the slow cooker and set aside (covered) to keep warm.

If you'd like to serve the pork roast with a sauce, strain the liquid from the slow cooker, discarding the onions, garlic, etc. Pour into a small saucepan and bring to a boil. Simmer until reduced and thickened, about 15-20 minutes. Meanwhile shred the pork roast.

Serve the roast with the sauce on the side, or as desired.