Heat the oven to 400F.
Place the sweet potatoes on a large, rimmed baking sheet. Drizzle with 2 tablespoons oil, sprinkle with ¼ teaspoon salt and toss to coat.
Place the onion, apples, mushrooms, and celery on a second large, rimmed baking sheet. Drizzle with 2 tablespoons oil, sprinkle with ¼ teaspoon salt and toss to coat.
Place both pans in the oven and roast for 20 minutes. Remove from the oven, flip the sweet potatoes, and stir the onion-apple-mushroom-celery mixture.
Place both pans back in the oven and cook for an additional 20 minutes, or until everything is golden and cooked through.
Scrape the onion-apple-mushroom-celery mixture onto the pan with the sweet potatoes. Add the pecans, cranberries, and sprinkle with sage (if using). Toss everything together, and serve.