Preheat your oven to 450 degrees F. Grease a 15x10-inch baking pan (see notes for a half-sheet pan) with butter. Rinse and peel your potatoes. Slice them thinly (1/8 -inch to 1/4-inch thick), using a food processor, mandoline, or the slicer on a box grater.
Cut off the root and stem ends of your onion and peel it. Cut it into thin rings by starting at one end and slicing downward. Continue slicing the rest of the onion into rings. Separate the rings and set them aside.
In a small bowl, combine the Parmesan, garlic powder and pepper. Melt your butter in a small pan on the stove or in a small bowl in the microwave.
To assemble, place some potatoes in a single layer, slightly overlapping, in your baking pan. Sprinkle on a third of your Parmesan mixture evenly. Add some onions on top in a single layer. Repeat with another layer of potatoes, the Parmesan mixture, and onions. For your third layer, add the potatoes, Parmesan mixture, onions, and drizzle your melted butter all over the top.
Bake uncovered for 30-35 minutes, until the top layer is browned and crispy.
Serve with salt at the table, for anyone who wants to add it.
Store leftovers in the refrigerator for up to five days.
If using a 13 x 18 half-sheet baking pan, you will probably need 8 or 9 medium potatoes to make three layers. Use a medium onion and 1.5 times the Parmesan mixture and butter.Slightly adapted from Better Homes and Gardens New Cook Book.