Lightly grease the insert of a 6-quart slow cooker with non-stick cooking spray.
Place the sliced onion, half of the frozen pierogies, and half of the sliced beef sausage into the slow cooker.
Repeat with the remaining pierogies and sausage. Scatter the cubed cream cheese over the top and sprinkle with 1 cup of the shredded cheddar cheese.
Pour the beef broth evenly over the mixture and season with black pepper.
Cover and cook on LOW for 3 to 4 hours, or until the pierogies are tender and hot.
Stir the mixture gently to blend the melted cream cheese and broth into a smooth sauce. Be careful not to break the pierogies.
Top with the remaining 1 cup of cheddar cheese. Cover and let sit for 5 minutes until the cheese is melted.
Garnish with fresh parsley and serve warm.