Add the chicken, onions, carrots, celery, leeks, mushrooms and garlic to a large roasting pan or 2 roasting pans.
Evenly drizzle the oil over the ingredients and mix until coated.
Roast in the oven at 425° for 45-60 minutes or until lightly browned.
Transfer the ingredients to a very large stockpot and place it on the cooktop.
Add in the thyme, parsley, bay leaves, and peppercorns and fill it up with water until the ingredients in the pot have been covered by 6 inches.
Simmer on low heat for 4-6 hours while skimming the impurities off of the top every 45 minutes or so.
Strain completely and reuse the bones and veggies for a remouillage or discard.
Cool the stock completely and store it.
Chef Notes:• There is no comparison of homemade chicken stock to store bought, it is significantly more flavorful.• Do not use bell peppers, spicy peppers, tomatoes, vegetables from the radish family or any overly strong flavored vegetables.• If you’re making a lot of stock be sure to use a couple different pans so that they can all get roasted.• It is said that chicken stock cannot extract much more flavor past the 6 hour mark.