3/4cupfull-fat plain Greek yogurt*about 7 ounces, or sour cream
For the Topping:
2cupscrushed cornflakessee blog post above for other options
Place a rack in the center of the oven and preheat the oven to 350°F. Coat a 9x13-inch casserole dish with nonstick spray.
Spread the defrosted hash browns onto paper towels or a clean kitchen towel. Pat as dry as possible.
In a Dutch oven or very large, deep skillet, heat the butter over medium heat. Once the butter has melted, sprinkle the flour over the top. Whisk and cook, stirring constantly, until the flour is golden brown and smells nutty, about 90 seconds (don’t shortcut this step, or you may have a raw flour taste).
Reduce the heat to low. Slowly add the milk and approximately half of the chicken broth, whisking constantly to avoid lumps.
Whisk cornstarch into the remaining broth until smooth. Add to the skillet and whisk to combine.
Whisk in the Worcestershire, poultry seasoning, salt, mustard, garlic powder, onion powder, and pepper.
Bring sauce to a simmer over medium high heat, whisking constantly. Continue to cook and stir until the sauce is thickened, about 7 to 8 minutes. Adjust the heat as needed so that you maintain a constant bubble but not an aggressive boil.
Remove the skillet from the heat. Whisk in 1 cup of the cheddar and Parmesan until smoothly combined. Whisk in Greek yogurt.
Add the dried hash browns and stir until evenly combined.
Transfer hash brown mixture to prepared baking dish and smooth into an even layer. Evenly top with the remaining 1/2 cup shredded cheddar.
Prepare the topping: Place the cornflakes in a small bowl and pour the melted butter over the top. With a spatula, stir until the corn flakes are evenly coated. Sprinkle over the top of the casserole.
Cover the casserole with foil. Bake for 40 minutes, then uncover. Return to the oven and continue baking until the casserole is hot and bubbly, about 12 minutes more.
Switch the oven to broil (keep the pan on the center rack). Broil until the topping is golden, about 1 to 2 minutes, watching the carefully the entire time to make sure it does not burn. Remove from the oven and let rest at least 10 minutes. Enjoy!
*I do not recommend fat-free Greek yogurt here, as it may curdle and will not make the casserole as rich and satisfying. If you want to go old school, you can replace it with sour cream. I typically purchase a 7-ounce sized small container (Fage sells one this size).
TO STORE: Refrigerate leftovers for up to 5 days.
TO REHEAT: Rewarm leftovers in a baking dish in a 350°F oven (best method) or gently in the microwave.
TO FREEZE: You can freeze this entire casserole either baked or unbaked for 3 months. Let thaw overnight in the refrigerator, then bake as directed.
MAKE AHEAD: Assemble the casserole to the point of baking up to 2 days in advance and refrigerate (wait to add the corn flake topping, as it can become soggy). When ready to bake, add the topping, then bake as directed.
TO USE FRESHLY GRATED POTATOES: Peel (optional) and grate the potatoes (you’ll need 6 cups or 2 pounds). Place them in a colander and rinse. Spread them onto a clean kitchen towel and pat very dry. Change out the towel and pat again to remove as much moisture as possible. Note that I think frozen potatoes really work best here.
HALVE IT. Recipe may halved and baked in an 8- or 9-inch square baking dish.