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 an even layer at the bottom of the slow cooker.
03 - Distribute half of the sliced kielbasa and half of the sliced onions over the pierogi layer.
04 - Repeat the layering with remaining pierogi, kielbasa, and onions.
05 - Whisk together sour cream, melted butter, cream cheese, garlic, black pepper, salt, and smoked paprika in a mixing bowl until completely smooth.
06 - Drizzle the creamy mixture evenly over the layered ingredients in the slow cooker.
07 - Sprinkle the shredded cheddar cheese uniformly across the top.
08 - Cover and cook on low heat for 4 hours until pierogi are tender and casserole is bubbly throughout.
09 - Top with fresh chopped chives just before serving, if desired.