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 layering with remaining pierogi, kielbasa, and onions to create a second complete layer.
04 - Whisk together sour cream, melted butter, cream cheese, garlic, black pepper, salt, and smoked paprika in a mixing bowl until completely smooth and well combined.
05 - Pour the creamy sauce mixture evenly over the layered ingredients, then sprinkle shredded cheddar cheese across the top surface.
06 - Cover and cook on low setting for 4 hours until pierogi are tender and casserole is bubbly throughout.
07 - Sprinkle fresh chopped chives over the top just before serving, if desired.