Preheat an oven to 450 °F. Put the parsnips, pears, and onions in bowl, drizzle with oil, sprinkle with ½ teaspoon salt, and pepper. Toss it well. Lightly grease the baking pan, and transfer the parsnips, pears, and onion to it. Bake for about 40 minutes, and stir after every 10 minutes.