Lightly grease the inside of your slow cooker with cooking spray or butter.

Place half of the frozen pierogi in a single layer at the bottom of the slow cooker. Evenly scatter half of the sliced smoked sausage and half of the chopped onion over the pierogi.

In a medium bowl, whisk together the cream of mushroom soup, milk, and sour cream until smooth and well combined. Season with salt and black pepper to taste.

Pour half of the prepared creamy sauce over the first layer of pierogi, sausage, and onion. Then, sprinkle half of the shredded cheddar cheese over the sauce. Repeat the layering process with the remaining pierogi, sausage, onion, sauce, and cheese.

Cover the slow cooker and cook on low for 4-6 hours or on high for 2-3 hours. The dish is ready when the pierogi are tender and cooked through, and the cheese is melted and bubbly.

Once cooked, gently stir the pierogi bake to combine the layers. Ladle generous portions into bowls and garnish with sliced green onions or crispy bacon bits, if desired. Serve hot and enjoy!