Prepare the ingredients for cooking.
Layer the bottom of the Crockpot with sauerkraut.
Add half of the frozen pierogi on top of the sauerkraut.
Sprinkle half of the onion, garlic powder, and smoked paprika over the pierogi.
Add half of the kielbasa slices over the onions.
Repeat the layering process with the remaining ingredients.
Top the casserole with shredded cheese and dollops of sour cream.
Cover and cook on low for 6 to 8 hours or until heated through.
Serve warm, garnished with additional sour cream if desired.