Preheat & Prep: Preheat your oven to 350°F (175°C) and lightly grease a 9×13-inch casserole baking dish.

Sauté Aromatics: Preheat a medium pot over medium heat on the stove-top. Add the olive oil, heat it through, and add the diced onions. Sauté the onions until they start to brown and soften.

Add Garlic: Smash and mince the garlic cloves and add them to the onions. Stir and sauté for about one minute until highly fragrant.

Create the Roux: Sprinkle the all-purpose flour over the onions and stir well. Stir constantly for a few seconds to cook out the raw flour taste.

Add Liquids: Start slowly pouring in the chicken stock while whisking or stirring continuously to prevent lumps. Pour in the milk immediately after the stock while stirring as well.

Thicken Sauce: Let the liquid mixture heat through and start to thicken into a creamy gravy.

Add Cheese & Seasoning: Remove from the heat. Stir in the sour cream, the 8 oz of shredded sharp cheddar cheese, salt, and fresh cracked black pepper. Let the mixture sit for a moment so the cheese melts into the thick sauce. Taste the sauce carefully to make sure you have enough seasoning, then stir in the minced parsley.

Combine: In a very large mixing bowl, gently toss the completely thawed hash browns with the warm cheese sauce until the potatoes are thoroughly coated.

Transfer to Dish: Spread the potato mixture evenly into the prepared 9x13 casserole dish. Make sure it is spread in a uniform, flat layer.

Bake: Spread the remaining 1 cup of shredded cheddar cheese evenly over the top. Bake uncovered for 35-40 minutes, until the casserole is beautifully browned and bubbling around the edges.