Brown the kielbasa and onions: Heat butter in a large skillet over medium heat. Add sliced kielbasa and cook until browned on both sides, about 5 minutes. Add onions and cook until softened, 3-4 minutes more.
Layer the casserole: Place half the frozen pierogies in a 6-quart slow cooker. Pour 2 cups chicken broth over them. Add half the kielbasa-onion mixture and 1 cup shredded cheddar. Repeat layers with remaining pierogies, broth, kielbasa, and cheese.
Cook: Cover and cook on HIGH for 2-3 hours or LOW for 4-5 hours, gently stirring once halfway through to prevent sticking.
Make the cream sauce: About 30 minutes before serving, warm cream cheese with 1/2 cup chicken broth in a small saucepan over low heat, whisking until smooth.
Finish the casserole: Pour cream cheese mixture over the casserole and gently stir to combine. Sprinkle remaining 1 cup cheddar on top. Cover and cook on LOW for 30 more minutes until cheese melts and sauce thickens.
Serve: Garnish with sliced green onions if desired. Let rest 10 minutes before serving for best texture.
