Spray a 6-quart slow cooker with nonstick cooking spray.
Place one package of frozen pierogies in the bottom of the slow cooker.
Top with half of the sliced kielbasa and 1 cup of shredded cheddar cheese.
Layer the second package of frozen pierogies on top.
In a bowl, combine the cream of chicken soup, chicken broth, chopped onion, garlic powder, and black pepper. Pour the mixture evenly over the pierogies.
Top with the remaining kielbasa and 1 cup of shredded cheddar cheese.
Cover and cook on low for 4-5 hours, or until the pierogies are tender and the cheese is melted.
Garnish with chopped green onions if desired before serving.