Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish.

Cook the frozen pierogies according to package instructions. Drain and set aside.

In a large skillet, cook the chopped bacon over medium heat until crispy. Remove the bacon with a slotted spoon and set aside, leaving the bacon drippings in the skillet.

Add diced onion to the skillet and cook until softened, about 3-4 minutes. Add minced garlic and cook for an additional 1 minute.

Sprinkle all-purpose flour over the onions and garlic, stirring constantly for about 1-2 minutes until the flour is lightly browned.

Gradually whisk in the milk, continuing to stir until the mixture is smooth and begins to thicken, about 5-7 minutes.

Remove the skillet from heat and stir in shredded sharp cheddar cheese, shredded mozzarella cheese, grated Parmesan cheese, salt, and black pepper until the cheese is fully melted and the sauce is smooth.

Arrange the cooked pierogies in the prepared baking dish. Pour the cheese sauce evenly over the pierogies.

Sprinkle the cooked bacon on top of the cheese sauce.

Bake in the preheated oven for 25-30 minutes, until the casserole is bubbly and golden brown on top.

Garnish with chopped green onions, if desired.

Serve the Cheesy Pierogi Casserole hot and enjoy!