I have been making what my daughter named pillow potatoes and vegetables for 25 years. Long before Reynolds came out with their cooking pouches. There was a cooking company who sold cookware and called it greaseless cookware. They would come to your house and cook ya a meal using their cookware using water only. They swore that steamed fried potatoes were great. Yeah, un huh NO! But I did get the idea about steaming potatoes a bit differently tho. It took about 3 different times till I got it right then I branched out into veggies.
I peel my potatoes and cut them in ¼ inch thick slices. I take 3 onion slices and separate them into rings. Then I start making the pillow part.
Take the two pieces of foil and put the shiny sides together and on one of the long sides, fold the edges together and roll over twice, making sure it is pressed together well. Then place it on cookie sheet and fold the top piece back.
Now put a single layer of potatoes on the bottom and then place a layer of onion over then sprinkle with the seasonings and keep layering taters and onions with seasons till all done. Now slice the butter into pats and place all over the top. Now bring top down and seal all edges, folding it at least twice but leave as much room as possible.
Place this in 400 degree oven, the steam will puff it up and it is ready about 10-15 minutes after puffed. Take this out and then open just one side and be careful cause the steam will burn ya. Layer the cheese slices over the top and reseal and place back in the oven for 10 minutes.
Notes; I have done this with all kinds of veggies, I of course change the seasonings depending on the veggie but I have done broccoli, cauliflower, carrots, sweet potatoes etc. The other thing I have done that my family loves is English peas, I add a touch of mint. I sometimes place cheese over them and the kids love cheesy peas.