Preheat your oven to 350°F and generously grease a 9x13 inch baking dish with butter or cooking spray. While the oven heats, finely dice your onion into 1/4-inch pieces and measure out all your ingredients. I like to melt the butter and let it cool to room temperature before mixing—this prevents the sour cream from curdling when you combine everything together.

In a large mixing bowl, combine the frozen hash browns (no need to thaw), cream of chicken soup, sour cream, and the cooled melted butter. Stir well to coat the hash browns evenly. Add the diced onion, 1.75 cups of shredded colby cheese, black pepper, and garlic powder. Mix everything thoroughly until the hash browns are evenly distributed and all ingredients are well incorporated—this ensures consistent flavor throughout the casserole.

Transfer the hash brown mixture from Step 2 into your prepared baking dish, spreading it evenly. Top with the remaining 3/4 cup of thickly shredded colby cheese, distributing it evenly across the surface. I prefer thick cheese shreds on top because they melt into golden pockets rather than creating a dense, uniform crust.

Place the casserole in your preheated 350°F oven and bake for 45-55 minutes, until the top is golden brown and the edges are bubbling. The exact time depends on your oven, so start checking around 45 minutes. The casserole is done when it's heated through and the cheese on top is melted and lightly browned.