Prepare the Crockpot by spraying it with non-stick cooking spray to ensure an easy release of the casserole later.

Layer half of the frozen pierogi at the bottom of the crockpot, followed by half of the sliced kielbasa and half of the chopped onion.

In a mixing bowl, combine the condensed cream of mushroom soup, sour cream, and melted butter; stir until well incorporated.

Spread half of the creamy sauce mixture over the layered pierogi, kielbasa, and onion.

Repeat the process by adding the remaining frozen pierogi, kielbasa, onion, and pouring the remaining creamy sauce over.

Cover the crockpot with its lid and set it to cook on low for 4-5 hours.

In the last 30 minutes of cooking, sprinkle the shredded cheddar cheese evenly over the top of the casserole.

Once cooking is complete, serve the casserole warm and garnish each serving with chopped chives.