Why This Recipe Works: Mashed potato casseroles transform the humble potato into a comfort-food side dish that’s more than the sum of its parts. We wanted to design our smooth, cheesy, crunchy-topped casserole so it could be made ahead and reheated for company without fuss.
Keeping the potatoes creamy required mixing them with a dairy product. Whole milk and butter were obvious additions that loosened the mash to the right consistency but weren’t enough to prevent dry, grainy potatoes after reheating. Sour cream added luxury that lasted and a refreshing tang. The cheddar cheese didn’t incorporate well when we stirred it into the mash, so we sprinkled a cheesy layer on top. And, to complete the dish, we sprinkled a homemade breadcrumb topping over the cheese.
This casserole was creamy and smooth after reheating and retained all its flavor. For a slightly spicy version, substitute pepper Jack cheese for the cheddar. For the smoothest texture, err on the side of overcooking the potatoes rather than undercooking them.
Serves 8 to 10
For the topping:
- 4 slices hearty white sandwich bread, quartered
- 2 tablespoons unsalted butter, melted
- 1/4 cup minced fresh parsley
For the casserole:
- 5 pounds russet potatoes, peeled and sliced 3/4-inch thick
- 2 1/2 cups whole milk, warmed
- 1 cup sour cream
- 8 tablespoons unsalted butter, melted
- 2 1/2 teaspoons Dijon mustard
- 1 garlic clove, minced
- 1/8 teaspoon cayenne pepper
- 8 ounces shredded extra-sharp cheddar cheese (about 2 cups)
For the topping: Adjust oven rack to middle position and heat oven to 300 degrees F. Pulse bread into crumbs in food processor, about 6 pulses. Toss crumbs with melted butter and spread over rimmed baking sheet. Bake, stirring occasionally, until golden and dry, about 20 minutes. Let crumbs cool, then toss with parsley and season with salt and pepper to taste; set aside. Increase oven temperature to 350 degrees F.
For the casserole: Meanwhile, place potatoes in Dutch oven and add water to cover by 1 inch. Bring to boil, then reduce to simmer and cook until potatoes are tender and paring knife inserted into potatoes meets no resistance, about 15 minutes.
Drain potatoes in colander. Set ricer or food mill over now-empty, but still warm, pot. Working in batches, process potatoes into the pot. Stir in milk, sour cream, butter, Dijon, garlic, and cayenne until uniform. Season with salt and pepper to taste.
Spread potato mixture into 13 by 9-inch baking dish. Sprinkle cheddar evenly over top, then sprinkle with breadcrumbs. Bake until casserole is hot throughout and crumbs are crisp, 25–30 minutes. Serve. (To make ahead, after sprinkling casserole with breadcrumbs, cover the dish tightly with plastic wrap and refrigerate for up to 2 days. To serve, remove plastic, cover with foil and bake for 20 minutes. Uncover and continue to bake until hot throughout, 20–30 minutes.)
Recipe courtesy of America’s Test Kitchen