Set the Instant Pot to high sauté to get it going. Add the chopped bacon and cook for about 10 minutes, or until it gets crispy.
Once the bacon is done, take it out and put it on a paper towel to drain the fat. Leave about 2 tablespoons of bacon fat in the pot and save the rest for another time. Do not clean or wipe out the pot. (There is so much flavor in the fond, and you don't want to waste it.)
Add the diced onions, celery, and a half teaspoon of salt to the bacon fat in the Instant Pot. Sauté for about 3 minutes, or until the onions become soft and translucent.
Add the apple cider vinegar and minced garlic, stir, and cook for another minute.
Be sure to scrape up all of the fond from the bottom of the pot. Pressure cookers will often give burn notices when anything is stuck to the bottom surface.
Now, add the potatoes, 2 teaspoons of salt, and the chicken broth to the pot. Mix everything together well.
Lock the Instant Pot's lid and set it to cook for 4 minutes at high pressure. Once the cooking time is up, use the quick-release method to let the pressure out of the pot.
While the pressure drops, make the cornstarch slurry. Mix the cornstarch and water in a small bowl until they are fully combined.
Once the pressure has gone down, remove the lid, change the Instant Pot back to the sauté setting, and add the cornstarch slurry. Stir constantly for about a minute until the soup thickens.
Set the Instant Pot to "keep warm" and carefully pour in the heavy cream. Mix gently until all of the cream has been fully incorporated into the soup.
Finally, mash the potatoes with a potato masher until the soup reaches your preferred consistency. Do not use a hand mixer; the soup will become overly starchy.
Ladle into bowls and serve topped with cheese, bacon, and green onions.