Preheat your oven to 350°F (175°C).

In a large skillet, cook the sausage over medium heat until browned and cooked through, about 5–7 minutes. Crumble as it cooks for even flavor distribution. Add chopped onion and cook until softened, about 3–4 minutes. Season to taste with salt and pepper.

In a large bowl, mix the cream of mushroom soup and sour cream until smooth. Stir in the cooked sausage and onion mixture, then gently fold in the frozen pierogi.

Pour half of the pierogi mixture into a greased 9x13-inch baking dish. Sprinkle half of the shredded cheese over the top. Layer the remaining pierogi mixture and add the remaining cheese on top.

Cover the dish with aluminum foil and bake in the preheated oven for 25 minutes.

After 25 minutes, remove the foil and bake for an additional 10–15 minutes, or until the cheese is bubbly and golden brown.

Let it cool for a few minutes before garnishing with chopped parsley. Serve warm.