Grills vary widely, so you may need to experiment with the heat setting. For my propane grill, I use medium-high heat, or even high heat, high enough to sear/brown the meat dish when I throw them on the grill.).
Wash potatoes and cut into slices 1/4 inch or so with skin intact.
Slice onion into thin slices, then separate into rings.
On heavy duty aluminum foil, about 18" to 24" long, put in the center a layer of potatoes (leaving plenty of foil on each side and each end to fold and seal the packet).
Follow with a layer of onion rings.
Sprinkle with salt, pepper, and a hint of garlic (powder, or freshly minced).
Repeat layers, dotting each second layer with butter.
Wrap the foil securely around the layers. Should make a packet about 15"-18" long and 10" wide.
Put the foil package on the grill about 45 minutes to an hour before the intended serving time. (These can also be prepared as described, but placing the packet in a stainless steel skillet on the stove top. Or in the oven. Be creative!).
It’s okay if the bottom layer “caramelizes;” some will love those bottom pieces!