Bring the cream cheese and butter to room temperature. Preheat the oven to 350°F/175°C.
Dice the celery and onion and sauté until very soft in about 1 Tablespoon of butter. Reserve the celery and onion to add to the potatoes after they boil.
Peel and cut potatoes and add them to a large pot of salted water. Bring the water to a boil and cook the potatoes until they are fork tender. Drain the water, reserving ⅓ of it to use for mashing if needed.
Add the cooked celery and onion, remaining butter, cream cheese, sour cream. Using a hand mixer or potato masher to mash the potatoes to your desired consistency. I like to keep some chunks in them. Add in some of the reserved cooking liquid (or chicken stock) if needed. Add the shredded cheese and mix to combine. Season with salt and pepper as needed.
Transfer the mashed potatoes to a casserole or baking dish and add your topping of choice. Bake uncovered for 20-25 minutes or until the topping is golden brown. Serve & Enjoy
Ninja Foodi or Instant Pot with Crisping Lid
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.
Potato Chip Topping
Combine all ingredients together and cover mashed potatoes. Bake according to the method you are using.
Frosted Flake Topping
Spread the chopped green onions over the top of the mashed potatoes. Mix the remaining ingredients together and spread over the mashed potatoes and green onions. Pour the bacon grease over the top if desired. Bake according the method you are using.
Nutritional Values shown using Frosted Flake topping. If you want to use both toppings, cut the ingredients in half and spread each topping over half of the mashed potato mixture. Increase the topping ingredients if using a large baking dish.