Preheat oven to 375°F (190°C). Grease a 9×13-inch baking dish.
Sauté Kielbasa: In a skillet over medium heat, cook kielbasa until lightly browned. Set aside.
Prepare the Mixture: In a large bowl, mix potatoes, onion, garlic, 1 ½ cups cheddar cheese, sour cream, soup, milk, paprika, salt, and pepper. Stir well.
Assemble the Casserole: Layer half of the potato mixture in the baking dish, then add kielbasa. Top with remaining potatoes.
Add Toppings: Sprinkle the remaining ½ cup of cheese. If using, mix butter with breadcrumbs and sprinkle for a crispy topping.
Bake: Cover with foil and bake for 40 minutes. Remove foil and bake another 15 minutes until golden and bubbly.
Garnish and Serve: Rest for 5 minutes, sprinkle with parsley, and enjoy!
