Preheat the oven: Set to 375°F (190°C). Grease a 9x13-inch baking dish with cooking spray or a thin layer of butter.

Sauté the sausage: In a large skillet over medium heat, cook the sliced sausage for 4–6 minutes, stirring occasionally, until browned at the edges. Transfer to a plate and leave 1 tablespoon of drippings in the pan.

Cook the onions and garlic: Add the butter to the same skillet. Stir in the sliced onion with a pinch of salt. Cook 6–8 minutes until soft and lightly golden. Add the garlic and cook 30–60 seconds until fragrant. Remove from heat.

Make the creamy sauce: In a bowl, whisk together sour cream, cream cheese, and chicken broth until smooth. Season with 1/2 teaspoon salt, 1/4 teaspoon pepper, and the paprika if using. The sauce should be pourable; add a splash more broth if it’s too thick.

Layer the casserole: Add half the frozen pierogi to the baking dish in an even layer. Top with half the sausage and half the onion mixture. Pour over half the creamy sauce and sprinkle with 1 cup of shredded cheese.

Repeat: Add the remaining pierogi, sausage, and onions. Pour over the rest of the sauce and finish with the remaining 1 cup of cheese.

Cover and bake: Cover the dish tightly with foil and bake for 25 minutes. This helps the pierogi steam and cook through.

Uncover and finish: Remove the foil and bake another 10–15 minutes, until the cheese is melted and the edges are bubbling. For a deeper golden top, broil for 1–2 minutes at the end—watch closely.

Rest and garnish: Let the casserole rest 5–10 minutes to set. Top with sliced green onions or chives before serving.