Combine the pineapple juice, brown sugar (don't forget the extra tablespoonful!), mustard, lemon juice, onion salt, and soy sauce in a medium saucepan, stirring well. Bring to a boil, reduce heat. Simmer, uncovered for 5 minutes; stir frequently. Remove sauce from heat, and set aside.
Trim any excess fat from the ham slices. Grill over medium coals for 20 to 25 minutes or until desired degree of doneness. While cooking, turn ham slices and baste every 5 minutes with the reserved sauce.
Grill the pineapple slices over medium heat for 1 minute on each side. Garnish each slice of ham with two pineapple slices, and serve with the remaining sauce.