Preheat the oven to 175°C (350°F) and lightly grease a 9×13-inch baking dish.
In a large mixing bowl, blend the cream of chicken soup with sour cream, 1½ cups of the cheddar cheese, chopped onions, garlic powder, black pepper, and paprika. Stir until the mixture is smooth and fully combined.
Add the thawed hash brown potatoes to the bowl and stir until the potatoes are evenly coated in the creamy mixture.
Gently incorporate the kielbasa slices into the mixture, folding them in to distribute evenly.
Transfer the entire mixture into the prepared baking dish, spreading it out in an even layer.
Sprinkle the remaining ½ cup of cheddar cheese evenly over the top.
If using the optional topping, combine the crushed crackers or cornflakes with melted butter and scatter over the cheese layer for added crunch.
Bake the casserole uncovered for 40 to 45 minutes, or until it is bubbling around the edges and the top is beautifully golden.
Allow the dish to rest for 5 minutes before serving.