Steps

1. Place your peeled and cubed potatoes into a large pot and cover them with cold water. Add a generous pinch of kosher salt. Bring the pot to a rolling boil over high heat, then reduce the heat slightly and cook until the potatoes are fork-tender, about 15 minutes. Drain thoroughly.
2. Transfer the hot, drained potatoes into a large mixing bowl. Mash them well with a potato masher or hand mixer until smooth. Stir in the sour cream, softened cream cheese, milk, onion salt, and black pepper. Mix until completely combined and creamy. Spread the mixture evenly into a large, lightly greased casserole dish.
3. Let the mashed potatoes cool completely in the casserole dish at room temperature for about 15 to 20 minutes. Once cooled, cover the dish tightly with aluminum foil and refrigerate. You can store it in the fridge for up to 2 days before baking.
4. When you are ready to bake, remove the casserole dish from the refrigerator. Let it sit out on the counter at room temperature for 30 minutes to take the chill off. While the potatoes rest, preheat your oven to 325 degrees F (165 degrees C).
5. Keep the casserole covered with the aluminum foil and place it into the preheated oven. Bake for 50 minutes, or until the potatoes are heated completely through and bubbling at the edges. Serve hot!