01 - Coat the inside of the Crock Pot with non-stick spray or a small amount of butter to prevent sticking.
02 - Layer half the frozen pierogi in the bottom of the Crock Pot, 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, combine sour cream, melted butter, cream cheese, garlic, black pepper, salt, and smoked paprika. Stir until completely smooth and well incorporated.
05 - Pour the creamy mixture evenly over the layered ingredients in the Crock Pot, then sprinkle the shredded cheddar cheese on top.
06 - Cover and cook on low for 4 hours, or until the pierogi are tender and the casserole is bubbly throughout.
07 - Garnish with fresh chives before serving, if desired. Serve hot directly from the slow cooker.