Step 1: Prepare the Hash Brown Crust
Preheat your oven to 400°F (200°C). Grease a 9×13 inch (23x33cm) baking dish. In a large bowl, combine the thawed hash browns, melted butter, garlic powder, onion powder, smoked paprika, salt, and pepper. Toss until the potatoes are evenly coated. Press this mixture firmly and evenly into the bottom and about 1 inch up the sides of the prepared baking dish, creating a solid crust. Pre-bake this crust for 20-25 minutes, until lightly golden and set. Remove from oven and reduce oven temperature to 375°F (190°C).

Step 2: Create the BBQ Chicken Filling
While the crust bakes, mix the shredded chicken with 1 cup of the BBQ sauce, olive oil, and chili powder in a bowl. Set aside.

Step 3: Prepare the Creamy Custard Layer
In a separate large bowl, whisk together the cream of chicken soup, sour cream, milk, beaten eggs, diced onion, dry mustard, and black pepper until the mixture is completely smooth and homogenous.

Step 4: Assemble the Bake
Spread the BBQ chicken mixture evenly over the pre-baked hash brown crust. Drizzle the remaining ½ cup of BBQ sauce over the chicken. Slowly and evenly pour the creamy custard mixture over the chicken layer, aiming to cover it completely. Gently shake the dish to help the custard settle into all the nooks.

Step 5: Add the Cheese Blanket
In a bowl, combine the shredded cheddar, Colby Jack, and Parmesan cheeses. Sprinkle this cheese blend evenly and generously over the entire surface. If using the optional crunchy topping, sprinkle it evenly over the cheese.

Step 6: Bake to Golden, Bubbling Perfection
Place the baking dish on a rimmed baking sheet to catch any drips. Bake at 375°F (190°C) for 35-45 minutes. The bake is done when the cheese is melted and spotty golden brown, the edges are bubbling vigorously, and the center is set (a knife inserted near the center should come out clean, and the internal temperature should reach 160°F / 71°C).

Step 7: Rest, Garnish, and Serve
Remove the casserole from the oven and let it rest for 15 minutes. This rest is crucial—it allows the custard layer to fully set, making it possible to cut clean squares. Garnish liberally with sliced green onions and fresh parsley or chives. Serve warm, with extra BBQ sauce or ranch dressing on the side for drizzling.