Lightly grease the inside of a slow cooker with nonstick cooking spray or a little butter.

In a medium bowl, whisk together the sour cream, cream of mushroom soup, chicken broth, melted butter, garlic powder, and black pepper until smooth.

Spread a thin layer of the sauce mixture over the bottom of the crockpot.

Add half of the frozen pierogi in an even layer over the sauce.

Top with half of the sliced kielbasa, half of the sliced onion, and 1 cup of the shredded cheddar cheese.

Repeat with the remaining pierogi, kielbasa, onion, and another 1 cup of cheddar cheese.

Pour the remaining sauce evenly over the top, then sprinkle with the remaining 1 cup of cheddar cheese.

Cover and cook on low for 4 to 5 hours, or until the pierogi are tender and the casserole is hot and bubbly. Avoid overcooking so the pierogi do not become too soft.

Turn off the slow cooker and let the casserole rest for 10 minutes before serving.

Garnish with chopped parsley if desired and serve warm.