Preheat the oven to 350 degrees F and grease a 9x13 baking dish with a bit of butter.
Melt 3 tablespoons of butter over medium heat in a large heavy-duty saucepan. Once melted, add the minced onion and cook until the onion has softened about 4-5 minutes. Be sure to stir often. Whisk the flour into the butter and onion mixture and cook, stirring constantly, for a full minute.
Slowly whisk in the chicken stock to the roux. Once the stock is fully incorporated, whisk in the milk and season with seasoned salt and 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 the cheese until smooth. Stir in the sour cream.
Add the thawed hash browns and gently fold the mixture together until well combined. Transfer the hash brown mixture to the prepared casserole dish.
Place the cornflakes into a large storage bag, and gently crush them with a rolling pin. Add the melted butter 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.