Layer halved onions, flat side down, on bottom of slow cooker. Combine brown sugar, cumin, coffee and paprika in a small bowl and rub mixture over pork. Place pork on top of onions. Add chicken stock and soy sauce. Pour half bottle of barbecue sauce over pork. Cover and cook on low for 7-8 hours, until pork is falling apart. Remove pork from slow cooker and, using two forks, shred into “strings” or small chunks. Return shredded pork to slow cooker, add remaining barbecue sauce, stir and cook, covered, for another 10 minutes. Serve on a bun and top with coleslaw.