Place the turkey carcass into a large slow cooker, fill the slow cooker with enough cold water to cover the bones (or until full) and select high for the heat. Set it to run for at least 8 Hours or overnight.
The next morning add the onion, carrot, celery, garlic, fresh herbs, peppercorns and bay leaves. Reduce the heat to low and simmer for an additional 4 Hours.
Turn off the slow cooker and strain the stock through a fine meshed sieve, discarding the solids and browned bits. For better clarity, line the sieve with cheesecloth first.
Let the turkey stock cool slightly, then skim the fat from the surface. Refrigerate in an airtight container or freeze.
For better flavor, roast the bones after you've carved and serve the turkey. This will help give your turkey stock a deeper flavor. For more information about roasting bones, go here.For more space to fit your aromatics or get a better yield from your turkey stock, break down the turkey carcass before placing them in the slow cooker. This will make the bones takes up less space, as well as make it easier to fit the lid.