Heat 2 tablespoons oil in the cooker.
Over high heat, brown the meat well on both sides, lifting it up frequently with tongs or spatula to prevent sticking.
Add additional fat if needed.
Set aside. (Don't be concerned if the brisket has to be squeezed against the sides of the pot; it will shrink considerably during cooking.)
In the oil remaining in the cooker, sauté the celery, carrot, and onion for 3 minutes, stirring occasionally and taking care to scrape up any browned bits of meat or vegetable sticking to the bottom of the pot.
Add the beef stock, water, cinnamon, nutmeg, and allspice.
Submerge the rack into this mixture, and place the meat on it with the fat side facing up.
Lock the lid in place and over high heat bring to high pressure.
Adjust the heat to maintain high pressure and cook for 55 minutes.
Let the pressure drop naturally, about 10 - 15 minutes.
Remove the lid, tilting it away from you to allow any excess steam to escape.
If the brisket is done, you should easily be able to pry a small chunk of meat from one end with a fork.
If it is not sufficiently tender, lock the lid back into place and return to high pressure for another 10 - 15 minutes.
If time permits cool to room temperature and refrigerate brisket and gravy overnight.
Remove any congealed fat and return the gravy to the cooker.
About 20 minutes before serving, stir in enough cider to equal approximately 2 cups.
Add the sweet potatoes, parsnips, and prunes.
Lock the lid in place and over high heat bring to high pressure, adjust heat and cook for 3 minutes.
Quick release the pressure. Add salt and pepper.
Cut the brisket into slices on the diagonal, place in the cooker, and spoon gravy and vegetables on top.
Cover but do not lock. Cook over medium heat just until the meat is warmed throughout.
Arrange on a platter and surround with the vegetables.
Pour gravy on top.
