Preheat your oven to 350°F. Lightly coat a 9x13 inch baking dish with cooking spray, making sure to cover the bottom and sides evenly to prevent sticking. While the oven preheats, finely dice your yellow onion into 1/4-inch pieces and set aside—this small dice will distribute the onion flavor more evenly throughout the casserole.

If your shredded hash browns were frozen, make sure they're fully thawed and squeeze out any excess moisture using a clean kitchen towel—this prevents a watery casserole. In a large bowl, combine the thawed hash browns with the kosher salt, garlic powder, and black pepper. Toss everything together thoroughly to ensure the seasonings are distributed evenly. I like to taste a small piece of hash brown at this point to make sure the seasoning level is right before adding the wet ingredients.

Pour the melted butter and condensed cream of chicken soup into the bowl with the seasoned hash browns, then add the diced onions. Using a spatula or wooden spoon, fold everything together gently but thoroughly, making sure the creamy mixture coats all the hash browns evenly. The butter and soup create the binding that makes this casserole rich and cohesive rather than dry.

Add the shredded Colby cheese to the hash brown mixture and fold it in gently with a spatula, being careful not to break up the potatoes too much—you want them to stay as intact as possible for better texture. Transfer the entire mixture into your prepared baking dish and spread it out into an even layer, pressing gently with the spatula so the top is fairly level. I prefer freshly shredded cheese over pre-shredded because it melts more smoothly and creates a creamier casserole.

Bake the casserole in the preheated 350°F oven for 30 to 35 minutes, until the top is golden brown and the edges are starting to crisp up. The casserole will bubble slightly around the edges when it's done, and a knife inserted into the center should meet no resistance. Let it rest for 5 minutes before serving—this helps it set slightly and makes it easier to scoop.