Combine the first 5 ingredients in a large zip-top plastic bag; seal and marinate in refrigerator 1 hour, turning twice. Remove parsnip and onion from bag, discarding marinade.
Preheat oven to 500°.
Place parsnip and onion in a shallow roasting pan coated with cooking spray. Drizzle with oil, and toss to coat. Sprinkle with salt and pepper. Bake at 500° for 30 minutes or until the parsnip is tender, stirring often.