Instructions

1. Arrange a rack in the middle of the oven and heat the oven to 400°F.
2. Let 30 ounces frozen shredded hash browns sit at room temperature while you prepare the sauce. The potatoes will not be completely thawed; that is OK. Dice 1 medium yellow onion and mince 3 garlic cloves. Grate 8 ounces sharp cheddar cheese on the large holes of a box grater (about 2 cups).
3. Melt 4 tablespoons unsalted butter in a Dutch oven or large pot over medium heat. Add the onion and cook, stirring occasionally, until softened, about 3 minutes. Add the garlic and cook until fragrant, about 30 seconds.
4. Add 1/4 cup all-purpose flour and stir to coat the onion until the flour is completely absorbed and no dry pockets of flour remain. Cook until golden-brown, 1 to 2 minutes.
5. While whisking constantly, slowly pour in 1 cup low-sodium chicken broth and 3/4 cup whole milk. Bring to a simmer over medium heat, whisking often, 1 to 2 minutes. Continue to cook, whisking often, until thickened enough to coat the back of a spoon, 1 to 3 minutes. Remove the pot from the heat.
6. Add the cheese, 8 ounces sour cream, 1 1/2 teaspoons kosher salt, and 1/4 teaspoon black pepper, and stir until well-combined and the cheese is mostly melted. Add the hash browns and stir to coat with the sauce. Transfer to a 9x13-inch baking dish and spread in an even layer.
7. Bake until the casserole is bubbling in the center, 40 to 45 minutes. Turn the oven to broil and broil until the top is lightly browned in spots, 1 to 2 minutes. Garnish with chopped fresh parsley leaves if using.