Foil pack potatoes are so delicious they should be illegal!!! My hubby makes these for me upon request.
You take potatoes whether they be Yukon gold, red bliss, Idaho, whatever your choice may be slice them up into roughly 1/4inch thick slices, try to keep them uniform, use a mandolin if you have one. Take an onion and cut in half. Slice one half up into roughly 1/4inch thick slices as well. Save the remaining half for another use.
Take foil large enough to fully wrap around the potatoes to create a package. Place potatoes still in a line in the foil and wrap the foil up around them to hold them in place. Section out the onion slices and portion between the potato slices, does not have to be one for one, it is used for flavor mainly. Do the same with the slices of butter. You do not need a lot of butter, maybe a tablespoon per pack. Close the foil up around the potatoes so the ends are folded up and so is the top. There might be a slight opening on the top, this is ok.
Place potato packs on a cookie sheet, this is recommended for oven cooking because you will need to pull them out to check them and if you are like me you will probably make a whole in the oil and all the butter will leak out and create a greasy mess and maybe a fire in your oven. Yes this has happened to me. So now I use the cookie sheet! Bake at 350 for roughly 45 minutes, check them and adjust cooking as needed. If cooking on a grill basically the same thing, place on the grates and cook until done, cover grill for best results. Just make sure that you start these first so they are fully cooked to be ready for meal time.