Preheat oven to 400F. Place onion, garlic, carrots, parsnips, turnips, burdock, thyme, sage, rosemary, olive oil, and salt in a large bowl; toss to combine. Divide between two baking dishes and roast for 1 hour, stirring every 20 minutes. Remove from oven and drizzle each pan with 1/2 cup water to deglaze. Scrape vegetables and any juice into a large pot, and add celery, bay leaves, parsley, mushroom stems, and remaining water; bring to a boil over high heat. Cover pot, reduce heat to low, and simmer for 1 1/2 hours. Remove from heat, strain stock, and compost vegetables. You should have 5 cups stock. Set aside, or if you are making this ahead of time, allow it to cool, pour it into jars, and then refrigerate until you're ready to use it. Stock will keep in the fridge for up to three days.