Remove most of the skin from the turkey carcass and place it in a large stockpot. Add the chicken wings (don't bother removing the skin from them).
Add the onion wedges, celery, carrots, garlic cloves, fresh parsley, fresh thyme, bay leaves, and peppercorns. Fill the stockpot with water until covering the carcass.
Cover with a tight-fitting lid and place in the oven. Simmer covered in the oven overnight.
In the morning, strain the mixture through a fine sieve. Discard the bones and veggies.
Pour the stock back into the stockpot and simmer on the stove for about 20-30 minutes, uncovered, to reduce a bit and get a richer flavor, make sure to skim off any fat from the top of the stock.
Pour the stock through a fine sieve (again) into storing containers. Allow the stock to cool then remove any remaining fat that floated to the top of the stock.
Once it has cooled, seal with a tight-fitting lid and refrigerate until needed. Enjoy!