Set your oven to preheat at 350°F (175°C). Pour the melted butter into a 9×13-inch baking dish, tilting the dish gently to coat the bottom evenly.
In a large bowl, combine the sour cream, condensed soup, 2 cups of the shredded cheese, diced onion, kosher salt, and ground black pepper. Stir until thoroughly mixed.
Gently fold in the frozen shredded hashbrown potatoes, ensuring all the pieces are evenly coated in the creamy mixture.
Transfer the mixture into the prepared casserole dish, spreading it out evenly. Top with the remaining 1 cup of cheddar cheese, distributing it uniformly over the surface.
Place the dish in the oven and bake for 45 to 50 minutes, or until the edges are golden and the center is hot and bubbly. Allow the casserole to rest for 15 minutes before serving.