Created by VegGirl on August 13, 2017
Step 1: Place potato halves in a large stockpot; add water to cover. Bring to a boil. Reduce heat; cook, uncovered, 20-25 minutes or until tender. Drain, then cool.
Step 2: 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.
Step 3: When the potatoes are cool enough to handle. Press through a potato ricer or strainer into a large bowl; stir in cheddar cheese, sour cream, mushroom mixture, onions and remaining butter. Transfer potato mixture to a greased 13x9-in. baking dish. Refrigerate, covered overnight or until your ready to bake it.
Step 4: 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.