In a large bowl, whisk together the cream of chicken soup, sour cream, milk, dried thyme, and black pepper until smooth and well combined. This is your sauce base.

Lightly grease the insert of a 5-quart or larger slow cooker.

Layer half of the frozen pierogi in the bottom of the slow cooker. Top with half of the sliced kielbasa and half of the chopped onion.

Carefully pour half of the prepared sauce mixture evenly over the first layer. Repeat the layering process: remaining pierogi, remaining kielbasa, remaining onion, and the rest of the sauce.

Cover the slow cooker and cook on LOW for 3.5 to 4 hours or on HIGH for 2 to 2.5 hours, until the pierogi are tender and heated through.

If using the cheese topping, about 15 minutes before serving time, sprinkle the cheddar cheese evenly over the casserole. Cover the cooker until the cheese is melted and bubbly. Drizzle with melted butter just before lifting the lid.

Stir gently to combine before serving. Serve hot.