Crispy Mashed Potato Casserole with Bacon, Cheese, and Scallions

Ingredients

• 5 pounds russet potatoes, peeled and cut into 3/4-inch cubes
• Kosher salt
• 1 stick unsalted butter, cubed and slightly softened
• 1 cup sour cream
• 1 cup heavy cream
• 6 thin slices bacon, diced
• 1 1/2 cups panko bread crumbs
• 1/4 cup thinly sliced scallions, white and light-green parts only
• 6 ounces grated cheddar cheese

Directions

1. Preheat the oven to 350 degrees F (177 degrees C). Put the potato cubes in a colander and wash them with cold water until the runoff is clear. Move them to a large saucepan, add enough cold water to cover them by at least 2 inches, and salt the water until it tastes very salty. Bring it to a boil over high heat, lower the heat to medium-low, and simmer until a knife slides into the potatoes without resistance, 10 to 15 minutes. Drain them in the colander, then rinse under hot running water for 30 seconds.
2. Arrange the potatoes in a single, even layer on 2 rimmed baking sheets. Place them in the oven until the extra moisture has cooked off and the outsides look dry, about 6 minutes.
3. If you are using a potato masher, put the hot potatoes in a large mixing bowl, add the butter and sour cream, and mash until just tiny lumps are left. Add the heavy cream and fold it in with a rubber spatula until combined. If you are using a ricer, rice the potatoes into a large mixing bowl, then fold in the butter, sour cream, and heavy cream until everything is incorporated. If you are using a food mill, run the potatoes and butter through the mill into a large mixing bowl, then add the sour cream and heavy cream and fold with a rubber spatula until combined. Season with salt.
4. Spoon the potatoes into a 9- by 13-inch baking dish and level the top. Cover with plastic and refrigerate for up to 5 days.
5. Two hours before you plan to serve, take the potatoes out of the refrigerator and leave them at room temperature for 1 hour. While they sit, preheat the oven to 350 degrees F (177 degrees C). Cook the bacon in a saucepan over medium-high heat until the fat has cooked out and the bacon is browned and crisp, about 6 minutes. Put the panko in a large mixing bowl, then scrape in the bacon along with its rendered fat. Mix thoroughly. Add the scallions and cheese, stir again, and season with salt. Scatter the bread crumb mixture over the potatoes.
6. Bake the casserole until the top is browned and the potatoes are hot all the way through, about 45 minutes. Serve.