01 - Coat the inside of the slow cooker 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 the remaining pierogi, kielbasa, and onions to create a second complete layer.
04 - In a mixing bowl, whisk together sour cream, melted butter, cream cheese, garlic, black pepper, salt, and smoked paprika until completely smooth and well combined.
05 - Pour the creamy mixture evenly over the layered ingredients in the slow cooker, then sprinkle the shredded cheddar cheese across the top.
06 - Cover and cook on low heat for 4 hours, or until the pierogi are tender and the casserole is bubbling around the edges.
07 - Sprinkle fresh chopped chives over the top before serving, if desired.