Preheat the oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish with butter or nonstick spray.
Cook the bacon: In a large skillet over medium heat, cook the chopped bacon until crisp and browned, 6–8 minutes. Transfer the bacon to a paper towel–lined plate, leaving about 1–2 tablespoons of rendered fat in the skillet. If not using bacon, add 1–2 tablespoons of butter or neutral oil to the skillet instead.
Sauté the aromatics: Add the sliced onion to the skillet with the bacon fat (or butter/oil). Cook over medium heat, stirring occasionally, until the onion is soft and lightly caramelized around the edges, about 8–10 minutes. Add the minced garlic and cook for 30–60 seconds, just until fragrant. Remove from heat.
Make the creamy sauce: In a medium bowl, whisk together the sour cream, heavy cream or half-and-half, melted butter, salt, pepper, and paprika until smooth. Stir in about 1 cup of the shredded cheddar cheese and the mozzarella cheese, if using, along with the chopped chives and parsley or dill.
Layer the pierogies: Arrange one bag of frozen pierogies in a single layer in the prepared baking dish. Scatter half of the sautéed onion-garlic mixture and half of the cooked bacon over the pierogies. Spoon or pour about half of the creamy sauce evenly over this layer.
Repeat the layers: Arrange the second bag of frozen pierogies on top. Add the remaining onion-garlic mixture and bacon, then pour the rest of the creamy sauce over everything, spreading it gently with a spatula so most of the pierogies are coated.
Top with cheese: Sprinkle the remaining 1/2 cup shredded cheddar cheese over the top. If you like a deeper golden crust, you can add a little extra cheese here.
Bake the casserole: Cover the dish loosely with foil and bake for 20 minutes. Remove the foil and continue baking for another 15–20 minutes, or until the sauce is bubbling, the pierogies are heated through, and the top is lightly browned and melty.
Rest and garnish: Let the casserole rest for about 5–10 minutes before serving; this helps the sauce thicken slightly and makes it easier to scoop. Garnish with additional chopped chives, parsley, or dill just before bringing it to the table.
Variations & Tips
• Vegetarian version: Omit the bacon and use butter or olive oil to sauté the onions. For extra savoriness, add a handful of sliced mushrooms to the skillet and cook them down with the onions, or sprinkle the casserole with smoked paprika or a dash of soy sauce to mimic some of the depth that bacon provides.• Different pierogi fillings: Frozen pierogies come in many varieties—potato and cheddar, potato and onion, or even sauerkraut and mushroom. This recipe works with most of them. Just keep in mind that very tangy fillings (like sauerkraut) will contrast more sharply with the creamy sauce, which can be delicious but a bit bolder; you may want to reduce the added salt slightly and taste as you go.• Lighter option: To make the dish a bit lighter, swap the heavy cream for half-and-half or even whole milk, and use light sour cream. The sauce will be slightly thinner but still comforting. You can also cut the cheese down by 1/2 cup and increase the fresh herbs to keep the flavor bright.• Add vegetables: For a more complete one-pan meal, tuck in some quick-cooking vegetables. Frozen peas or corn can be scattered between the pierogi layers straight from the freezer. Blanched broccoli florets or sautéed spinach also work well; just make sure any high-moisture vegetables are drained so the sauce doesn’t become watery.• Flavor twists: To lean into a more American casserole profile, add a teaspoon of Dijon mustard and a pinch of cayenne to the sauce for gentle heat. For a stronger Eastern European character, use a mix of sharp white cheddar and a smoked cheese, and finish the dish with a spoonful of sauerkraut or pickled onions on each serving.• Make-ahead and reheating: You can assemble the casserole up to a day in advance, cover tightly, and refrigerate. When baking from cold, add 10–15 minutes to the covered baking time to ensure the pierogies heat through. Leftovers reheat well in a 350°F (175°C) oven, covered, for about 20 minutes, or in the microwave in individual portions. If the sauce thickens too much on reheating, a splash of milk or cream will loosen it nicely.
