This is the "go-to" mashed potato recipe in my family. It can be put together up to 48 hours in advance, which makes it one of my favorites since the prep work is already done. If you're prepping this early, store in the fridge. Let sit for 30 minutes or so before cooking. Also, pealing the potatoes is optional and based on personal preference.
Preheat the oven to 325 degrees F (165 degrees C).
Place potatoes in a large pot of lightly salted water. Bring to a boil, and cook until tender, about 15 minutes. Drain, and mash.
In a large bowl, mix mashed potatoes, cream cheese, sour cream, milk, onion salt, and pepper. Transfer to a large casserole dish. Cut up the stick of butter and add "chunks" across the top of the potatoes. The butter will melt while cooking, giving the potatoes an extra buttery flavor.
Cover, and bake for 50 minutes in the preheated oven.