Use a sharp boning knife to remove the breasts, drumsticks, and thighs from chicken. Set aside. Remove the wings, and cut the carcass in half through back-bone; place wings and both halves of carcass in a 6- to 8-quart stovetop pressure cooker or electric multifunction cooker, such as an Instant Pot. Add water to barely cover. Bring to a boil over high (adjust the sauté function on an Instant Pot or use a similar setting on other multifunction cookers). After foamy scum appears, remove from the heat, and turn off the cooker. Drain chicken parts; discard liquid. Rinse chicken parts with cold water, and return to cooker. Add reserved breasts, drumsticks, and thighs, chicken feet, onion, apple, bay leaf, parsley, salt, and 7 1/2 cups water.
Secure pressure cooker lid. Heat over medium-high to bring to high pressure, and cook for 40 minutes. Remove from heat, and naturally depressurize for 25 minutes. (In a multifunction cooker, lock on the lid, and program the machine to cook on high pressure for 40 minutes. Turn off, and let depressurize for 25 minutes.)
Release residual pressure on pressure cooker. Remove and discard large solids from stock mixture. Set a fine wire-mesh strainer lined with paper towels, cheesecloth, or muslin over a pot, and pour stock through strainer. (There should be some fat remaining for flavor. If there’s too much fat for your taste, refrigerate overnight and remove the solidified fat the next day.) The stock will keep in the refrigerator for up to 5 days or frozen for up to 3 months.