Be sure to cut off the greenery sprouting from the tops of carrots and beets, as it saps nutrients, moisture, and sweetness from the root.
Preheat oven to 400°F. In a bowl, toss carrots, parsnips, beets, and onion with olive oil; season with salt and pepper.
Spread vegetables in a single layer on roasting pan, leaving as much space as possible between each. Roast until lightly browned and just tender (50-60 minutes), stirring halfway through baking. Remove; squeeze orange juice over top. Toss and transfer to platter. Serve with parsley.