Prep and cook potatoes - Scrub, peel, and dice potatoes in about 1 inch pieces. Add to a pressure cooker with one cup of water and salt. Put the lid on and seal the valve. Cook on high pressure for 10 minutes. Quick-release valve. Drain water off of potatoes. With the heat off, let potatoes sit in the pot for a couple of minutes to allow some of the water to evaporate.
Mash - Mash potatoes with a hand masher, or with an electric mixer. Be careful to not over mash potatoes or they will be gummy.
Mix potatoes - Whisk egg and add heavy cream and add to potatoes. Add cream cheese, sour cream, butter, seasonings, one cup of cheddar cheese, and half of the Parmesan cheese. Stir to combine until cream cheese is melted.
Prepare to Bake - Spread potatoes in a greased dish, then spinkle remaining cheese over the top.
Bake or Refrigerate for later - Bake covered at 350 degrees for 30 minutes, uncover and bake for 10 more minutes. OR refrigerate for later. When you're ready to bake them, let the potatoes sit at room temperature for about 30 minutes. Then bake for as instructed above.