In a medium saucepan over medium heat, melt 1/4 cup salted butter. Once melted, add in the finely diced 1 medium white onionhite onion and cook, stirring occasionally, until softened and lightly browned, about 5 minutes.
Sprinkle in 1/4 cup all-purpose flour, stirring continuously for about 2–3 minutes, until the mixture is smooth and turns a deep golden color.
Gradually pour in 1 cup chicken brothStir in 1 cup heavy cream and 1 cup sour creamuntil the sauce is smooth and fully combined. Lower the heat to a simmer and cook for an additional 2–3 minutes, stirring occasionally.
Stir in 1 tablespoon chopped fresh parsley, 1 tablespoon garlic powder, 1 teaspoon hot sauce, 1 teaspoon black pepper, and 1 teaspoon salt. Remove from heat and set aside.
Potato Casserole
Preheat the oven to 350°F (175°C). Lightly grease a 9x13-inch or 3-quart baking dish.
Using a grater or food processor, grate 6 large russet potatoes. Rinse the grated potatoes in a colander under cold water and squeeze out any excess moisture. Transfer the potatoes to a large mixing bowl.
Pour the prepared sauce over the grated potatoes. Add 1 cup of the shredded cheddar cheese and gently stir until well combined.
Spread the potato mixture evenly in the prepared baking dish. Sprinkle the remaining 1 cup cheddar cheese and 1 cup crushed corn flakes over the top. Drizzle 2 tablespoons of melted butter evenly over the casserole.
Bake in the preheated oven for about 1 hour, or until the potatoes are tender and the top is golden brown and crispy. Serve hot.
Notes
Short on time? Use frozen shredded hashbrowns in place of the russet potatoes. You'll need 3 pounds, or 48 ounces.