You will need 2 large pieces of heavy duty aluminum foil.
Spray the bottom layer of the foil packet heavily with non-stick spray.
Scrub potatoes well and slice fairly thick.
Layer in a slightly overlapping way on top of one of the pieces of foil, but not piled up in a bunch.
Sprinkle the onion and garlic over the potatoes.
Season liberally with salt and pepper.
Layer slices of butter all over the potatoes.
Sprinkle the Parmesan, Mozzarella and Cheddar cheese on the potatoes.
Lay the other piece of foil over the potatoes and fold all the edges in all the way up to the potatoes inside the packet. You don't want the top layer of foil to be touching the cheese inside too much so do not mash it down.
With a small knife or scissors cut 3 tiny holes in the top center of the foil packet.
Lay packet on a hot grill and cook over medium heat for about 30 minutes.
Remove and let sit for about 5 minutes.
Be careful when tearing off the foil top because steam could burn you.