Step 1

Preheat oven to 450°. Divide turkey parts, onions, carrots, celery, and garlic between 2 rimmed baking sheets. Drizzle with oil, season with salt and pepper, and toss to coat. Roast, turning once, until vegetables are soft and caramelized (they should stick slightly to pans), 45–50 minutes.

Step 2

Transfer to a large pot and add thyme, bay leaves, peppercorns, and 16 cups water. Pour ¼ cup water into each baking sheet, scraping up any browned bits; add to pot. Bring to a simmer and cook until stock is deep golden brown and reduced to about 12 cups, about 1½ hours. Strain into containers. Let cool; cover and chill.

Step 3

DO AHEAD: Stock can be made 3 days ahead. Keep chilled, or freeze up to 3 months.