Place the ham in a large pot or slow cooker, and fill with enough water to cover. Bring to a boil, then reduce the heat to low, and simmer for 8 to 10 hours. Remove the meat from the water, and allow to cool. If it has cooked long enough, it will fall into pieces as you pick it up.
Pull the ham apart into shreds once it is cool enough to handle. It doesn't have to be tiny shreds. Place the shredded ham into a slow cooker. Stir in the sliced onion, mustard and brown sugar, cover, and set to Low. Cook just until heated. Serve on dinner rolls with dill pickle slices. It's also good with coleslaw, on the sandwich or on the side.