Peel potatoes and cut into large chunks. Add potatoes to a large pot of cool water and parboil them, or bring to a boil for about 2 minutes. Drain all but about an inch of water.
Thinly slice the onions and mix in with the potatoes. Add oil, salt and pepper and combine.
Place in a baking dish and bake at 400° for 30 minutes. Turn them and bake for an additional 20-30 minutes or until edges are crispy and a fork easily goes through them.