Preheat oven to 450°. In a large mixing bowl, toss together apples, parsnips, sweet potatoes, onion, oil, salt, and pepper. Arrange evenly in an 11×17-inch pan with an edge. Place in lower third of oven and roast for about 45 minutes, stirring vegetables and rotating pan every 15 minutes. When everything is fork-tender and nicely browned (some may be more tender than others; not to worry), remove from oven and set aside until cool enough to handle.