01 - Coat the inside of the Crock Pot with non-stick spray or a small amount of butter to prevent sticking.
02 - Arrange half the frozen pierogi in the bottom of the slow cooker, then top with half of the sliced kielbasa and half of the sliced onions.
03 - Repeat with remaining pierogi, kielbasa, and onions to create a complete second layer.
04 - In a mixing bowl, combine sour cream, melted butter, cream cheese, garlic, black pepper, salt, and smoked paprika. Whisk until completely smooth and well incorporated.
05 - Pour the creamy mixture evenly over the layered ingredients, then sprinkle shredded cheddar cheese across the top.
06 - Cover and cook on low heat for 4 hours, until pierogi are tender and the casserole is bubbling throughout.
07 - Sprinkle fresh chopped chives over the top before serving, if desired.