Prepare 6 sheets of foil 12 inches long.
Build your sandwiches by dividing the ham between the rolls with 1 slice of cheese on each.
Place 2 sandwiches onto each piece of foil.
Whisk together the butter, mustard, onion and brown sugar. Pour evenly over the sandwiches.
Fold the sides of each packet up and seal closed.
Place on warm coals, turn frequent, and cook 10-15 minutes or until cheese is melted and bread is lightly toasted. (Cook on the grill on low heat or in the oven at 350°F for 15 - 20 moinutes.)