Preheat oven to 450 degrees, with rack in lower third. Place parsnips, carrots, bacon, and thyme in a single layer on a rimmed baking sheet.
Drizzle with syrup; season with salt and pepper. Toss well to combine.
Bake until bottoms of vegetables begin to caramelize and turn dark brown, about 20 minutes. Remove from oven, and toss vegetables carefully. Return to oven, and bake until all sides are well browned, about 25 minutes more. Discard bacon and thyme. Serve immediately.