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Layered Mashed Potato & Mushroom Casserole

  • 1 h 20 m
  • 2 h
EatingWell Test Kitchen
“Fancy up regular mashed potatoes with a layer of mushroom duxelle—a sauté of finely chopped mushrooms and shallots. We omitted the traditional butter in the duxelle and added chard for a nutritional boost. Serve this hearty side in place of mashed potatoes at any holiday feast or enjoy it as a vegetarian main dish. ”


    • 3½ pounds potatoes, preferably Yukon Gold, peeled and cut into 1-inch pieces
    • 1½ pounds cremini or white mushrooms, halved
    • 1 tablespoon butter
    • ½ cup nonfat buttermilk
    • 1 large egg plus 1 large egg white, beaten
    • 1¼ teaspoons salt, divided
    • 1 tablespoon extra-virgin olive oil
    • ¾ cup chopped shallots
    • 3 cloves garlic, finely chopped
    • 4 cups trimmed and finely chopped chard or spinach
    • 1 cup mushroom broth or reduced-sodium beef broth
    • 2 tablespoons all-purpose flour
    • ½ teaspoon freshly ground pepper
    • ½ teaspoon chopped fresh rosemary
    • 1 cup freshly grated Parmesan cheese, divided


  • 1 Bring 1 inch of water to a simmer in a large pot. Place potatoes in a steamer basket, cover and steam over medium-low heat, replenishing the water as necessary, until the potatoes are fall-apart tender, 20 to 30 minutes.
  • 2 Meanwhile, working in two batches, place mushrooms in a food processor and pulse, stopping to scrape down the sides as needed, until the mushrooms are coarsely chopped.
  • 3 Transfer the potatoes to a large bowl. Add butter and mash until chunky-smooth. Gradually stir in buttermilk, egg and egg white and 1 teaspoon salt. Set aside.
  • 4 Heat oil in a large skillet over medium heat. Add shallots and garlic and cook, stirring, until fragrant and beginning to soften, about 1 minute. Add the mushrooms and cook, stirring often, until they release their liquid and the pan is almost dry, 10 to 12 minutes. Add chard (or spinach) and continue to cook, stirring, until wilted, about 4 minutes. Whisk broth and flour in a small bowl. Add to the pan along with the remaining ¼ teaspoon salt, pepper and rosemary. Cook, stirring, until the mixture bubbles and thickens, about 1 minute.
  • 5 Preheat oven to 400°F.
  • 6 To assemble, spread half of the mashed potatoes in an even layer in a 9-by-13-inch (or similar 3-quart) baking dish. Sprinkle half the Parmesan over the potatoes. Spread the mushroom mixture on top and spread the remaining potatoes over the mushroom layer. Top with the remaining Parmesan.
  • 7 Bake until hot throughout and the top is golden brown, about 35 minutes.
  • Make Ahead Tip: Assemble through Step 6, cover and refrigerate for up to 1 day. Bake, uncovered, in a 400°F oven until hot, 50 minutes to 1 hour. To reheat, bake, covered, at 350° until hot, 40 to 50 minutes.
  • Keep food fresh: If you’re storing food in your fridge for a few hours or more, it’s best to keep it in an airtight container or in a container covered tightly with foil. Foil is best at creating a barrier that doesn’t let unwanted flavors in (or out) while you store your food.
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