Let’s dive into making this delightful casserole:

1. Preheat your oven to 350°F (175°C).
2. In a large skillet, melt the butter over medium heat. Add the onion and cook until it’s soft and translucent, about 5 minutes.
3. Add the sliced kielbasa to the skillet and cook until it’s browned on the edges, stirring occasionally.
4. In a large mixing bowl, combine the cooked onion and kielbasa with the sour cream, milk, and half of the shredded cheddar cheese. Season with salt and pepper to taste.
5. Grease a 9×13-inch baking dish. Layer the frozen pierogi evenly across the bottom of the dish.
6. Pour the kielbasa mixture over the pierogi, ensuring they are evenly covered.
7. Sprinkle the remaining cheddar cheese over the top.
8. Cover the dish with aluminum foil and bake for 20 minutes. Then, remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.
9. Allow the casserole to cool slightly before serving. Garnish with chopped parsley if desired.

Cooking has always been about the journey — the little steps that lead to a delicious outcome. Remember, it’s okay to adjust as you go, just like I learned in my grandmother’s kitchen.