Preheat oven to 350°F and grease a 9x13-inch baking dish with butter or cooking spray.
In a large mixing bowl, combine cream of chicken soup, sour cream, 1 1/2 cups cheddar cheese, diced onion, minced garlic, garlic powder, smoked paprika, black pepper, and salt. Mix until smooth.
Add thawed hash brown potatoes to the bowl and stir until every piece is coated in the cheesy sauce.
Fold in sliced kielbasa sausage gently, making sure it's evenly distributed throughout the mixture.
Pour the entire mixture into your prepared baking dish and spread evenly. Sprinkle the remaining 1/2 cup cheddar cheese over the top.
For extra crunch, mix melted butter with crushed cornflakes and scatter over the cheese layer (optional).
Bake uncovered for 40 to 45 minutes until the casserole is bubbly around the edges and the top is golden brown.
Let it rest for 5 minutes before serving. Garnish with sliced green onions for a fresh finish.