This dish can be made with only carrots or only parsnips (instead of both) by using three pounds of either.
Preheat oven to 450 degrees. Toss carrots, parsnips, and shallots with oil, and season with salt and pepper. Spread mixture onto 2 baking sheets, and roast, turning sheets twice and rotating once, until vegetables are golden brown and tender, about 35 minutes (remove shallots if cooked first). Serve with olive relish.