Place the sliced kielbasa and frozen pierogi into the bottom of a lightly greased slow cooker.
In a separate bowl, whisk together the cream of chicken soup, sour cream, milk, half of the shredded cheddar cheese, onion, dill weed, and black pepper until smooth.
Pour the soup mixture evenly over the pierogi and kielbasa in the slow cooker. Stir gently to coat everything.
Cover the slow cooker and cook on low for 4 to 5 hours or on high for 2 to 3 hours, until the pierogi are tender.
Stir the casserole gently before serving. Sprinkle the remaining 1/2 cup of cheddar cheese over the top during the last 15 minutes of cooking, or just before serving.
Notes
You can substitute smoked sausage for kielbasa if needed.
For a richer flavor, use cream of mushroom soup instead of cream of chicken soup.
If you prefer a thicker sauce, reduce the milk to 1/4 cup.
This recipe works well with potato and onion pierogi or sauerkraut pierogi.