Step 1
Place potatoes in a large pot of lightly salted water. Bring to a boil and cook until tender, about 15 minutes; drain.
Step 2
Transfer potatoes into a large bowl and mash. Stir in sour cream, cream cheese, milk, onion salt, and pepper until wellcombined. Transfer into a large casserole dish.
Step 3
Let potatoes cool completely, 15 to 20 minutes. Cover with aluminum foil and refrigerate for up to 2 days.
Step 4
When ready to bake, remove casserole from the refrigerator and let sit for 30 minutes on the counter. Preheat the oven to 325degrees F (165 degrees C).
Step 5
Bake, covered, in the preheated oven for 50 minutes.
0 servings