Preheat the oven to 350 degrees. Sprinkle a baking sheet with salt. Top with onions, carrots, quince, and celery roots in a single layer. Cover with aluminum foil, and bake until just tender, about 1 hour.Advertisement
Remove from oven, and uncover. When cool enough to handle, peel and core the quince. Cut vegetables into serving-size pieces. Transfer the quince and vegetables to a medium bowl. Drizzle with oil, and toss to evenly coat. Place in a baking dish just large enough to hold them in a single layer. Top with herb sprigs.
Shortly before serving, preheat the oven to 400 degrees. Place baking dish in oven, and cook until heated through, 10 to 15 minutes.