My Polish neighbor brought this dish to the potluck, and the pan was licked clean in minutes!

Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch white ceramic baking dish so the pierogies don’t stick.
Arrange the frozen pierogies in a single, snug layer in the bottom of the baking dish. It’s fine if they overlap just a bit, but try to keep them mostly in one layer so they bake up evenly and get those nice golden tops.
Scatter the sliced kielbasa rounds evenly over the pierogies, making sure every scoop will get a bit of sausage. Let some pieces rest on top so they can crisp around the edges in the oven.
In a medium bowl, whisk together the heavy cream and the condensed cream of mushroom soup until smooth and well blended. This makes the rich, creamy sauce that will bubble up around the pierogies as they bake.
Pour the cream-and-soup mixture evenly over the pierogies and kielbasa, tilting the pan gently if needed so the sauce seeps down between everything. You want all the dumplings to have some sauce around them.
Sprinkle 1 1/2 cups of the shredded sharp cheddar cheese evenly over the top of the casserole, reserving the remaining 1/2 cup for later. This first layer will melt into the sauce and help it thicken and cling to the pierogies.
Cover the baking dish tightly with foil and place it on the middle rack of the preheated oven. Bake for 25 minutes, until the sauce is hot and starting to bubble around the edges.
Carefully remove the foil from the dish. Sprinkle the remaining 1/2 cup of cheddar cheese over the top. Return the casserole to the oven, uncovered, and bake for another 15–20 minutes, or until the cheese is fully melted, the top is golden in spots, and the kielbasa edges are lightly crisped.
If you’d like a deeper golden brown top, switch the oven to broil for 1–3 minutes at the end, watching closely so the cheese bubbles and browns without burning. The pierogies should be soft and tender underneath, with a creamy, thickened sauce.
Remove the casserole from the oven and let it rest for about 5–10 minutes before serving. This short rest helps the sauce settle so every spoonful has soft dumplings, smoky sausage rounds, and gooey, bubbling cheese.
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