Peel and clean potatoes and cut into chunks. Place in a large pot and cover with cold water. Bring pot to a boil and cook at a simmer until potatoes are tender, 10-20 minutes.
Drain potatoes with a colander and place into the bowl of a stand mixer. Gently beat potatoes with the paddle attachment. While mixing, add butter, milk, onion salt, and black pepper. Cut cream cheese into chunks and slowly add, making sure it fully mixes in. Add sour cream. Mix another 30 seconds, until everything is smooth.
Scoop mashed potatoes into a shallow casserole dish. If making ahead, allow to cool, then cover and refrigerate. They will keep for a several days. To serve, bake in a preheated 350 degree oven for 45 minutes, until heated all the way though and top browns slightly. Makes lots. Enjoy!