Preheat oven to 350°F. Grease a 9×13-inch pan with cooking spray. Microwave the hash browns in the bag for 30 seconds, then place them in a strainer to thaw while preparing the rest of the dish.: You should notice the oven warming and the air filling with a neutral, warm scent, which primes the kitchen for baking. The brief microwave blast loosens ice crystals so the hash brown potatoes separate easily, and straining allows excess moisture to drip away; this prevents a watery casserole. A good visual cue is that the shredded potatoes look loose rather than clumped. If they feel icy or soggy, let them sit a little longer in the strainer to shed water. Common mistakes include skipping the straining step, which can lead to a runny final texture, or over microwaving, which partially cooks them and alters the desired bake timing.
Melt 6 tablespoons butter in a large pot over medium-high heat. Add onions, garlic, and seasonings (start with 1-1/4 teaspoons salt and 1/2 teaspoon pepper, adjusting to taste). Cook, stirring occasionally, until onions soften (about 3–5 minutes). Sprinkle flour over the mixture, stirring constantly, for 1 minute to prevent browning. Gradually whisk in milk, followed by chicken broth, to form a smooth sauce.: As the butter melts, it creates a glossy pool that carries the heat evenly, releasing a nutty aroma when the onion softens. Stirring the flour in removes the raw taste and forms the base for a velvety sauce. When you add milk and chicken broth gradually, whisking prevents lumps and results in a homogenous mixture that will coat a spoon. You want to see a smooth, slightly thickening sauce at this stage. A frequent slip is adding the liquids too quickly, which can create lumps, so whisk steadily and keep the heat moderate to avoid scorching.
Bring the mixture to a boil, stirring frequently. Continue stirring for 3 minutes at a boil, then reduce to a simmer and cook until thickened, about 3 more minutes (see note 5). Remove from heat and let cool slightly. Stir in Cheddar cheese, 1/4 cup at a time, until melted. Mix in sour cream until smooth.: At a boil, the sauce will become lively and glossy, and you should hear a light bubbling sound that indicates the starches are activating. Stirring for the full time ensures the sauce darkens to a rich hue and achieves body. Reducing to a simmer lets the sauce stabilize and thicken without breaking. When you add the Cheddar cheese in small increments, each addition melts evenly and you avoid clumps; the mixture should become silkier with each stir. Folding in sour cream at the end brings a gentle tang and velvety finish. A classic error here is adding cheese too quickly or on too high a heat, causing oil separation or grainy texture, so temper the sauce with gentle heat.
Add the thawed hash browns to the sauce and stir to coat. Transfer the mixture to the prepared pan. Cover with foil and bake for 20 minutes.: When you fold the slightly cooled sauce into the hash brown potatoes , the sensory change is immediate, the mixture becoming cohesive and glossy. Transfer to the greased pan with purposeful, even spreading so the casserole bakes uniformly. Tenting with foil traps steam and warms the center gently, allowing flavors to meld. After 20 minutes you should sense a steaming, bubbling surface beneath the foil. Avoid packing the pan too tightly, which prevents even heat circulation and can cause a mushy texture in the center.
While the potatoes bake, crush potato chips into coarse crumbs (you’ll need 2 cups). Combine with Parmesan cheese in a bowl. Melt the remaining 2 tablespoons butter, drizzle over the topping, and stir to coat.: Crushing the kettle cooked potato chips by hand gives you variable textures, some larger shards for crunch and finer crumbs for coverage. Mixing in Parmesan cheese adds savory depth to the topping, and the melted butter encourages browning and helps the crumbs adhere. The aroma will be buttery and slightly toasty once combined. A misstep is pulverizing the chips too finely, which removes contrast, or skipping the butter , which reduces browning and richness.
Remove foil from the pan. Evenly sprinkle the topping over the potatoes. Bake uncovered for an additional 15–20 minutes, until golden brown and bubbling around the edges. For extra crunch, broil for 1 minute. Let stand 10 minutes before serving.: When you remove the foil you should see steam lift and the casserole surface beginning to firm. The topping will toast and release a fragrant, salty crunch, while the edges will bubble aesthetically, signaling doneness. Broiling for a brief minute intensifies the crisp without overcooking the interior, but keep a close watch as it can change from perfect to burnt in seconds. Letting the dish rest allows the sauce to settle and makes serving cleaner. A common oversight is slicing straight from the oven which causes the casserole to fall apart, so allow the recommended rest time.