Ingredients
5 pounds potatoes (about 6 large)1/2 cup butter, divided1/2 cup chopped fresh mushrooms2 tablespoons all-purpose flour1 cup 2% milk or half-and-half cream1 teaspoon salt1/2 teaspoon pepper1/2 teaspoon dried thyme2 cups shredded sharp cheddar cheese2 cups sour cream1 medium onion, chopped1/2 cup dry bread crumbs1/4 cup grated Parmesan cheese
Preparation
Place whole potatoes in a large stockpot; add water to cover. Bring to a boil. Reduce heat; cook, uncovered, until tender, 20-25 minutes. 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 until thickened, 1-2 minutes.
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, until heated through, 55-65 minutes.
