Add sliced onions to the bottom of a 5-6 quart slow cooker bowl. Place the pork shoulder on top of the onions. Sprinkle with seasonings and garlic. Pour apple cider over the pork shoulder and seasonings. Cover and cook on high for 4-5 hours, or low for 8-10 hours. Pork is ready when the internal temperature reaches at least 145°F. Carefully remove pork from the slow cooker. Shred by hand using forks. 

If desired, return shredded pork to slow cooker to absorb the juices. Serve with cooked onion slices. 

Alternately, shredded pork may be served in your favorite sandwich roll with homemade barbecue sauce, red onions and pickles.