Preheat oven to 425°F.
Roast the bones. Add 1 Tablespoon oil to a large roasting pan. Add bones and roast on the lower rack until deep golden brown, about 40–45 minutes, stirring occasionally for even browning.
Roast the vegetables. While the bones roast, toss onions, carrots, celery, and leek with the remaining 1 Tbsp oil on a large baking sheet. Roast until browned, about 30 minutes.
Simmer the stock. Transfer the roasted bones to an 8-quart stockpot. Add the cold water and bring to a boil. As soon as it boils, reduce heat to low and skim off any scum that rises to the top. Add roasted vegetables, thyme, peppercorns, and bay leaves.
Keep it gentle. Simmer uncovered for 2 hours. The liquid should barely bubble—if it boils (or if you stir), the stock can turn cloudy.
Strain and chill. Strain through a cheesecloth-lined strainer and discard vegetables and bones. Refrigerate and remove the fat from the top. Use immediately or freeze in small containers for later.