Slice the onions and place them in the base of the slow cooker. Tear the sage leaves and scatter a few in with the onions.
Place the joint of pork on top, season and add a few more sage leaves on top.
I cook in the slow cooker for two hours on high then six hours on low, total eight hours cooking time. If all on low I would do cook for about ten hours for a large joint of pork. (Reduce the time to eight hours on low for a smaller joint.)
To make the crackling, transfer the pork to an oven tray, pat the fat dry with kitchen towel and rub it with salt, and put it in a hot oven (220 degrees) for 20-30 minutes.