Lightly spray a (25 x 18-inch) sheet of heavy duty foil with cooking spray. Cut the loaf of French bread into twelve 3/4-inch slices, cutting to within 1/4-inch of the bottom, being careful not to cut all the way through.
In a small bowl, stir together butter and mustard. Spread on every other slice of bread creating 6 sandwiches. Fold a slice of cheese in half diagonally and tuck into each sandwich. Divide ham evenly among sandwiches, tucking into fit. Place the loaf in the middle of the foil. Cover and tightly seal the edges.
Place wrapped loaf on the grill over medium heat. Cover grill; cook for 20-25 minutes or until cheese is melted and the loaf is hot, turning loaf frequently. Open packet carefully, to allow stem to escape. Remove foil from loaf and pull apart sandwiches.