Scatter onion slices in the bottom of a slow cooker insert; set aside.
In a small bowl, mix together spice rub: cumin through cayenne. Rub pork shoulder evenly with spice mixture then place on top of onions. Pour in beer or broth then turn slow cooker on Low for 8 hours or High for 4 hours.
Before serving, carefully shred pork with two forks, discarding any large chunks of excess fat. Serve as is or in tacos, sandwiches, pasta, salads, soups and more.