Toss celery root with oil, pepper and salt and spread on a rimmed baking sheet. Roast until starting to brown, 10 to 12 minutes. Add apples, toss gently and continue roasting until the apples and celery root are tender, 6 to 10 minutes more.
Meanwhile, cook bacon in a medium skillet over medium heat, stirring occasionally, until just crispy. Remove to a paper-towel-lined plate with a slotted spoon; discard all but 2 teaspoons of the bacon fat. Add maple syrup to the fat in the pan and bring to a boil, scraping up the browned bits. Add the cooked bacon and thyme (or rosemary). When the celery root and apples are tender, gently toss them with the maple-bacon glaze and roast for about 5 minutes more.