Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish with melted butter.

In a large mixing bowl, combine thawed frozen shredded hash browns, shredded cheddar cheese, diced onion, diced bell pepper, and cooked crumbled bacon or sausage.

In a separate bowl, whisk together sour cream, melted butter, milk, eggs, salt, and black pepper until smooth.

Pour the wet mixture over the hash brown mixture and gently fold until just combined.

Spread the combined mixture evenly into the prepared baking dish, smoothing the top.

Bake in the preheated oven for 45-50 minutes, until the edges are golden brown and the center is firm.

Let the casserole cool for about 5 minutes before serving.