Lay the slabs of onion on the bottom of a slow-cooker crock. Lay the pork shoulder on the onions, fat side down. Sprinkle the salt over the pork roast and rub it in, flipping it to fat side up. Pour the boiled cider syrup over the pork roast, put the lid in place, and cook on low for 12 hours.

Use tongs to transfer the pork to a large mixing bowl. Be careful, as the pork will want to fall apart and may splash back into the hot pan juices.

Strain the pan juices into a 2 quart saucepan and let stand for 10 minutes to allow the fat to rise to the top. Skim as much of the fat from the pan as possible and then reduce the sauce by 2/3. While it is boiling, use your hands or two forks to shred the pork, removing the bone and as much fat as possible. Transfer to a casserole or serving dish tented with foil to keep warm.

When the pan juices have reduced, whisk in the honey and Dijon mustard before pouring over the pork and tossing. Serve immediately or cool and divide into meal sized portions in air-tight containers and freeze for up to 3 months.