Cut two pieces of foil twice the size of the ham slice. Put ham slice on one side of one piece of foil after you spread both sides of the ham slice with mustard. Sprinkle brown sugar on the top side and add pineapple chunks with pineapple juice. Fold foil into an envelope, crimping all edges securely. Repeat with second foil layer. Cook 6 minutes a side on top of charcoal gray coals.