Preheat the oven to 350℉ (175℃) and grease a 9x13 baking dish with a bit of butter.
Melt 3 tablespoons unsalted butter over medium heat in a large heavy-duty saucepan. Once melted, add ½ large yellow onion (minced) and cook until the onion has softened about 4-5 minutes. Be sure to stir often. Once softened, whisk ¼ cup all-purpose flour into the mixture, stirring constantly, for a full minute.
Slowly whisk in 1 ½ cups low-sodium chicken stock to the roux. Once the stock is fully incorporated, whisk in 1 cup milk and season with 1 ½ teaspoon seasoned salt and ½ teaspoon ground black pepper.
Bring the mixture to a boil over medium-high heat. Once boiling gently, reduce the heat to medium-low and simmer, stirring frequently, until the mixture is slightly thickened, about 5 minutes. Remove from the heat and stir in 2 cups shredded sharp cheddar cheese until smooth. Stir in the ½ cup sour cream.
Add 26 ounces shredded or diced hash browns (thawed) and gently fold the mixture together until well combined. Transfer the hash brown mixture to the prepared casserole dish.
Place 3 cups cornflakes into a large storage bag, and gently crush them with a rolling pin. Add 2 tablespoons unsalted butter (melted) to the bag and toss to combine the cornflakes with the butter. Sprinkle this topping evenly over the casserole dish.
Bake for 40-45 minutes, until hot and bubbly around the edges. Let sit for 5-10 minutes before serving.
Notes
Keep it Vegetarian-Friendly. Replace the chicken broth with vegetable broth for a delicious vegetarian-friendly hashbrown potato casserole. Change Out the Topping: Use butter crackers, such as Ritz Crackers in place of the cornflakes.Storage: Allow the casserole to cool to room temperature. Tightly cover the dish or transfer the leftovers to an airtight container and store in the refrigerator for up to 3 days and reheat individual servings on a heat-safe dish.