Dice the onion and celery into ⅛-¼" dice. Add them to the inner pot.
Peel and dice the potatoes and place them in the inner pot of the Ninja Foodi or Instant Pot. Add the chicken stock, salt, pepper, onions, celery, butter, cream cheese, and sour cream.
Put the pressure lid on and turn the valve to seal. Set the pressure on high for 7 minutes. When the time is up, allow the pot to natural release for 5 minutes, then manually release the remaining pressure. I will warn you, it is going to look terrible! The cream cheese and sour cream might look curdled or split. Don't worry, it comes back together just fine.
Use a potato masher that is safe for the inner pot or you can transfer the contents to a mixing bowl if you want to use an electric mixer in the non-stick pot. Once mashed to your desired consistency, add the shredded cheese and stir. Taste for seasonings and add more salt and pepper if desired.
Choose the topping you want and spread it over the top of the mashed potatoes in the inner pot. Close the crisping lid and set bake/roast (or air fry if you are using a different crisping lid and don't have bake/roast) at 300°F/150°C for 15 minutes or until golden brown.
You can also transfer the potatoes to a baking dish and bake in the oven.