We make this dish all the time, and it is so easy. If you don't do it already, you must try it.
Slice a bunch of potatoes very thinly. Slice an onion thinly as well. Take a big sheet of foil and spray it with olive oil (or brush some on if you don't have an olive oil sprayer). On ONE HALF of the foil, layer on potatoes, onions, salt, pepper and more olive oil. You can get crazy here with variations. The potatoes pictured above are made with white truffle salt and a red onion. I've added green and red peppers, and also garlic on occasion. Whatever you want goes. As you create the packet, it will look like this:
When you are done layering, fold the foil in half and pinch it closed so that no air can get in. It should look like this:
Cook over indirect heat on the grill for up to an hour.
When you unwrap the packet, be VERY careful. The steam inside gets crazy hot, and I've burnt myself a few times. You might want to even poke a few holes in the packet before unwrapping to prevent this.