Spray the slow cooker with nonstick cooking spray.
Get a long sheet of tin foil (about 1½ - 2 feet), scrunch it up and curve it into a rack. Place "rack" in the slow cooker.
Liberally season pork all over with salt and pepper.
In a small bowl, mix mustard, sage, marjoram and garlic together.
Use your clean hands to spread all over the pork (on all sides, and working it into the crevices).
Place pork on the “rack” in the slow cooker (fat side up if possible).
Cook on low for 8-10 hours.
Remove from slow cooker, let stand for 15 minutes then shred with two forks.
Serve over polenta.