Spray 13x9-inch pan with cooking spray; set aside. In 4-quart Dutch oven, place potatoes; add enough water just to cover potatoes. Cover and heat to boiling; reduce heat. Simmer covered 20 to 25 minutes or until potatoes are tender; drain. Shake potatoes in Dutch oven over low heat to dry.
Mash potatoes until no lumps remain. Add cream cheese, sour cream, salt and pepper; mix until well blended. Spoon into pan. Cover tightly with foil and refrigerate at least 8 hours but no longer than 24 hours. (To bake immediately, drizzle with butter and bake 30 minutes, adding onions for last 15 minutes of baking.)
Heat oven to 350°F. Drizzle butter over potatoes. Cover and bake 30 minutes. Sprinkle onions over potatoes. Bake uncovered 15 to 20 minutes longer or until potatoes are hot.