Make-Ahead Creamy Potatoes Recipe

Place whole potatoes in a large stockpot; add water to cover. Bring to a boil. Reduce heat; cook, uncovered, 20-25 minutes or until tender. Drain. Meanwhile, in a large saucepan, heat 1/4 cup butter over medium-high heat. Add mushrooms; cook and stir 2-4 minutes or until tender. Stir in flour until blended; gradually whisk in milk, salt, pepper and thyme. Bring to a boil, stirring constantly; cook and stir 1-2 minutes or until thickened. Peel and cube potatoes when cool enough to handle. Press through a potato ricer or strainer into a large bowl; stir in cheddar cheese, sour cream, mushroom mixture, onion and remaining butter. Transfer potato mixture to a greased 13x9-in. baking dish. Refrigerate, covered, overnight. Preheat oven to 375 °. Remove potatoes from refrigerator; uncover and let stand while oven heats. Sprinkle with bread crumbs and Parmesan cheese. Bake, uncovered, 55-65 minutes or until heated through.