I use two pieces of foil. The one the potatoes are on is greased or sprayed. Wrap the first on in a package then put on another piece to protect it. All I do is thinly slice potatoes and place in the foil add slice onions and fresh mushrooms and butter the whole thing and put in my favorite spices and plenty of parmesan cheese. After that I place them on the grill and grill for 20-40 min depending on thickness of the slices. When they are tender I open them up and put shredded cheese in them and let it melt. Take off and serve.