Place the pork in a slow cooker. Arrange the onions around the meat. Top with the mustard and all of the seasonings. Cover the slow cooker and cook on high for 4 hours or low for 8 hours until the pork is very tender. Shred the meat with 2 forks and set aside until ready to use.

When you are ready to make the sandwiches, preheat the broiler to high and spread the bread with butter. Broil the bread butter-side up until toasted and golden. Remove from the oven and reduce the heat on the oven to 425F. Layer the bottom of each bread section with cheese, ham, pickles, some of the pork mixture, and more cheese. Top with bread and wrap in tin foil. Repeat with the remaining sandwiches. Bake the wrapped sandwiches for 10 minutes to melt the cheese, then unwrap and devour. Enjoy!