Add stock, balsamic, potatoes and onions to the crockpot, and toss to coat.
Mix softened butter with rosemary and garlic in a small bowl.
Pat pork dry with paper towels.
Coat pork in butter mixture, then season on all sides with salt and pepper.
Place pork on top of potatoes and cover.
Cook on high for 3 hours, or until roast reaches 145 degrees. Alternatively, you can cook on low for 4-5 hours.
Once pork reaches an internal temperature of 145 degrees f with a thermometer inserted in the center, remove it to a cutting board, and allow it to rest for 10 minutes, then slice.
While pork rests, taste potatoes for seasoning, and add salt as needed.
Make a slurry of 2 tablespoon cornstarch and 2 tablespoon water, whisk well until no lumps are present, and stir into the potatoes to make a gravy.
Serve sliced pork with potatoes and gravy.