Place the onion, carrot, celery, chicken carcass, garlic cloves, and peppercorns into a large pressure cooker. Add water until it reaches 2/3 of the way up the pot (or until you reach the maximum fill line, according to your pressure cooker's instructions) and seal the lid.
Bring the pressure cooker up to full pressure over high heat. Reduce the heat to medium-low and cook, maintaining full pressure, for 30 minutes.
Remove the pot from the heat and allow the pressure to drop naturally. Do not use quick-release methods.
When the pressure has dropped, remove the lid. Strain the stock into a bowl and discard the chicken bones and vegetables. Let the stock cool to room temperature, and then transfer to the refrigerator (see Editor's Note). When the stock is cold, any fat will have risen to the surface and solidified. Skim the fat off completely, and season to taste with sea salt.
