1. Preheat oven to 400 degrees. Arrange turkey pieces in large roasting pan. Roast until well-browned, about 1½ hours. Transfer pieces to a large stockpot.
2. Set roasting pan over two burners. Add water and bring to a boil, scraping up browned bits from bottom. Pour liquid into pot.
3. Add onion, carrot, celery, peppercorns, bay leaf, parsley, and thyme. Cover with cold water to a depth of 1 inch.
4. Bring to a boil over medium-high heat. Reduce heat, partially cover pot and simmer until bones fall apart and vegetables are very soft, 2 to 2½ hours. From time to time, skim off foam and debris that collects on top.
5. Strain stock, gently pressing on solids to extract liquid.
6. Rinse out stockpot and pour strained liquid back into it. Simmer uncovered until stock reduces to about 2 quarts, about 1 hour longer. The stock should be deeply flavorful and be the color of weak tea.
The stock can be made up well in advance. Let stock cool uncovered, transfer into glass jars with tight-fitting lids, and refrigerate for up to 4 days. Bring the stock to a simmer before using.
The stock can be frozen in airtight freezer bags or canning jars with tight-fitting lids for up to 3 months. Thaw in the refrigerator instead of the microwave. Depending on how you plan to use the stock, it might be most useful to divide the stock into 2- or 4-cup containers. Liquids expand when frozen, so leave 2 inches of headspace at the top of the container.