Salt and pepper the pork loin roast well on each side. Place the onion halves on the bottom of the slow cooker and place the loin roast on top of the onions. Pour apple cider into the bottom of the slow cooker. (You can substitute broth if you don't have cider on hand.)
Combine applesauce, dijon, maple syrup, rosemary, and apple cider vinegar and whisk until blended. Pour the mixture over the pork loin and cover the slow cooker with the lid.
Cook on low for approximately 6-8 hours or until a thermometer reads 160 degrees.
Note: Cooking times vary because slow cookers vary widely. And cooking times will vary based on the size of your roast. The general rule of thumb for pork loin in the slow cooker is 1 1/2 hours per pound of meat. Use that rule to determine when you should check your loin. Enjoy!