Preheat your oven to 350°F and butter a 9x13-inch baking dish thoroughly. Set aside while you prepare remaining ingredients—this prevents sticking and adds richness to the bottom layer.

In a large skillet over medium heat, melt 2 tablespoons butter and sauté diced onions and bell peppers until softened, about 5 minutes. These vegetables release their moisture and sweetness, creating flavor throughout the casserole.

In a large mixing bowl, whisk together eggs, heavy cream, whole milk, garlic powder, black pepper, salt, and paprika until completely combined. Pour this custard mixture into the prepared baking dish, creating your binding agent.

Layer frozen hashbrowns directly into the custard, breaking apart clumps gently with your fingers as you distribute them evenly. Scatter sautéed vegetables and smoked turkey strips across the hashbrowns, then sprinkle both shredded cheddar and mozzarella over everything uniformly.

In a small bowl, combine panko breadcrumbs with remaining 2 tablespoons melted butter and fresh chives, then sprinkle this mixture over the top layer. This topping creates the golden loaded hashbrown casserole's signature crunch and beautiful presentation.

Bake uncovered for 40 minutes until the casserole is bubbling at the edges and the breadcrumb topping turns golden brown. A knife inserted in the center should come out clean, indicating the egg custard has set properly.

Remove from oven and let rest for 5 minutes before serving. This resting period allows the custard to firm up slightly, making neat portions easier to cut and serve.