Spray the inside of a 6-quart slow cooker with cooking spray or line it with a slow cooker liner.

Place half of the frozen pierogies in an even layer across the bottom.

Scatter half of the sliced kielbasa and half of the chopped onion over the pierogies.

Sprinkle 1 cup of shredded cheddar evenly over that layer.

Repeat with the remaining pierogies, kielbasa, onion, and the rest of the cheddar to make two layers.

In a medium bowl, whisk the cream of mushroom soup and sour cream until smooth. Pour the mixture evenly over the layered ingredients.

Season lightly with salt and several grinds of black pepper. Cover and cook on low for 4 to 6 hours, until the pierogies are softened and heated through and the cheese is fully melted. Check at 4 hours — cooking times vary by slow cooker.

Turn off the slow cooker and let the casserole sit for 5 minutes before serving. Garnish with chopped parsley.