To cook over the fire, it’s best to place a grill over top. Spray a 25×18-inch sheet of heavy-duty aluminum foil with cooking spray. Cut the bread into 12 even slices, being careful not to cut all the way through to the bottom. Stir the butter and mustard together and spread on every other slice of bread. This will make about 6 attached sandwiches. Fold each slice of cheese in half and tuck into each of the 6 sandwiches. Divide the ham up evenly amongst the sandwiches, keeping it contained within the slices. Place the loaf in the center of the sprayed foil and seal the edges. Grill over the fire for about 10 minutes, turning it once in a while to prevent burning.