Follow these simple steps to create your Pierogi Breakfast Casserole:

1. Preheat the Oven: Preheat your oven to 350°F (175°C).
2. Prepare the Baking Dish: Grease a 9×13-inch baking dish with cooking spray or olive oil.
3. Sauté the Onion: In a skillet, heat olive oil over medium heat. Add diced onion and sauté until translucent, about 5-7 minutes.
4. Prepare the Egg Mixture: In a large bowl, whisk together the eggs, milk, sour cream, salt, and pepper until well combined.
5. Layer the Ingredients: Place half of the pierogi in the greased baking dish. Pour half of the egg mixture over the pierogi, then sprinkle half the cheese on top.
6. Repeat the Layers: Add the remaining pierogi, followed by the rest of the egg mixture. Finish with the remaining cheese.
7. Bake: Place the baking dish in the preheated oven and bake for 30-45 minutes or until the cheese is bubbly and golden brown.
8. Cool and Garnish: Once baked, take the casserole out of the oven and let it cool for a few minutes. Garnish with fresh chives if desired.
9. Serve: Slice into squares and serve warm.

By following these steps, you’ll create a nourishing and hearty casserole that embodies the flavors of traditional pierogi with minimal effort.