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