Step 1

If using frozen hash browns: Thaw 1 (32-ounce) package frozen shredded hash brown potatoes in the refrigerator overnight.

Step 2

If using whole potatoes: Halve 3 unpeeled medium russet potatoes lengthwise. Place in a Dutch oven or medium pot and add enough cold water to cover them about 1 inch. Add 2 tablespoons apple cider vinegar and 1 tablespoon of the kosher salt. Bring to a boil over high heat. Reduce the heat to maintain a simmer and cook until the potatoes are knife tender, about 10 minutes.

Step 3

Using tongs, transfer the potatoes to a baking sheet. Let cool to room temperature, about 30 minutes. Transfer to a zip top bag or airtight container and refrigerate overnight. (Alternatively, freeze the cooled potatoes on the baking sheet uncovered for 30 to 45 minutes. They won’t grate quite as easily as the refrigerated potatoes.)

Step 4

Grate the cut side of the potatoes on the large holes of a box grater until you get to the skin (it’s okay if some of the skin gets grated). Discard the skins.

Step 5

Arrange a rack in the middle of the oven to 350ºF. Coat a 9x13-inch baking pan with unsalted butter.

Step 6

Dice 1 medium yellow onion (about 1 1/2 cups) and mince 2 garlic cloves. Grate 8 ounces extra-sharp cheddar cheese on the large holes of a box grater (about 2 cups).

Step 7

Melt 4 tablespoons of the unsalted butter in a Dutch oven or large saucepan over medium-high heat. Add the onion and sauté until softened and translucent, 5 to 7 minutes. Add the garlic and cook until fragrant, about 1 minute.

Step 8

Sprinkle 1/4 cup all-purpose flour over the onion and garlic and stir until all of the flour is hydrated. Continue to cook, stirring constantly, until the flour looks and smells lightly toasted, 2 to 3 minutes. While whisking constantly, slowly pour in 2 cups low-sodium chicken broth and 1 cup whole milk. Bring to a gentle simmer. Simmer, stirring frequently and reducing the heat as needed, until the sauce is thickened and coats the back of a spoon, 5 to 7 minutes.

Step 9

Remove the pot from the heat. Add the cheese, remaining 2 teaspoons kosher salt, 1 teaspoon powdered ground mustard, 1 teaspoon black pepper, and 1/2 teaspoon cayenne pepper. Stir until the cheese is melted. Add 1 cup sour cream and stir to combine. Taste and season with more kosher salt and black pepper as needed. The sauce should be just a little bit saltier than you want the funeral potatoes to be.

Step 10

Add the potatoes and stir until each piece is evenly coated with sauce. Taste and season with kosher salt and black pepper as needed. Transfer to the baking dish and spread into an even layer.

Step 11

Place 4 cups cornflakes in a large zip top bag. Crush with your hands into small pieces. Melt the remaining 4 tablespoons unsalted butter in the microwave or on the stovetop. Pour into the bag and shake until the cornflakes are evenly coated. Sprinkle over the potatoes in an even layer.

Step 12

Bake until the cornflakes are just beginning to brown, 45 to 60 minutes. Let cool for 10 to 15 minutes before serving.