(If using premade or store-bought stock, skip to "FOR THE SOUP.") Add the chicken carcass to the insert of a slow-cooker or Instant Pot with onion, garlic cloves, bay leaf, rosemary, peppercorns. Add very hot water, 2 cups at a time, to the container in which the chicken was packaged to rinse any drippings. Add the water and drippings to the cooker. Repeat four times, for a total of 8 cups. Add one additional cup of water. Cook at least 8 hours (and up to 24) in the slow-cooker on low. FOR THE INSTANT POT: close the lid and seal the valve. Press the "Pressure Cook" button enough times so that "More" is selected. Press the plus sign until the cooker's display shows 60 minutes. After a brief pause, The Instant Pot will now pressurize for around 20 minutes before the clock begins to count down. When the cook time is complete, allow the pressure cooker to depressurize naturally (don't release the valve; this can take another 20 or so minutes).