Place 1 onion sliced in bottom of crock pot. Place pork shoulder roast (fat side up), on top of onions. Pour 1 bottle of beer over roast. Top roast with remaining onion slices. Cover, and cook on low for 6 - 8 hours. Remove roast, and using a couple of forks, shred pork into strands. Pour out excess liquid (preserving onions). Return shredded pork and onions to crock pot, add bar-b-cue sauce, stir to mix. Serve on buns. Enjoy!