Scrub your potatoes and slice each potato into 1/2-inch rounds, but keep them together in the potato shape.
Place one potato on each foil square and top with one onion round each, and then the butter pat on top of the onion (the size of your pat of butter is up to you, but mine are about 1 1/2 tablespoons each.
Now the fun part. I line up all of my spices in front of me and shake on as much as I like to each potato. Be very liberal--potatoes can take a lot of flavor.
Then put each foil square in front of you with the potato lengthwise. Grab the front flap and the back flap and pull them together over the potato and then roll them down together until you get close to the potato. Then grab the sides and roll them upward so that none of the butter spills out during baking.
Place them on a baking sheet in a 350-375° oven for about an hour, depending on how big your potatoes are and how thick you sliced them. You can check their doneness by sliding a knife into one through the foil--if it goes in smoothly, it's done.