Coat a large 6-quart slow cooker with nonstick spray. Line the bottom with the thinly sliced onions (2 medium or 1 very large). Pour in the apple juice and minced garlic.
In a small bowl, whisk together the brown sugar, chili powder, paprika, garlic powder, onion powder, mustard powder, ground cumin, salt, and pepper (I add 1 tablespoon salt and 1 heaping teaspoon cracked pepper). Remove pork from packaging and dab all over with a paper towel. Rub the spices into the pork -- all sides.
Place the pork on top of the onions; if there is a white layer of fat, place that facing up (the fat will render down and make the pork more tender).
Cover, and cook for 8-11 hours on low setting (a 5.5 pound pork is always done around 9 hours for me); I don't recommend cooking on high.
Once the meat is completely cooked through and shreds really easily, drain the liquid. Pull off the layer of fat on top (it should just easily pull off), and pull out the bone and discard both (or save the bone and the drained liquid to make homemade stock!)
Using two forks, shred the pork.