1. Begin by melting the butter in a skillet over medium heat. Add the chopped onion and sauté until golden and fragrant, about 5 minutes.
2. In a large bowl, mix together the sour cream, cream of mushroom soup, milk, and Dijon mustard until smooth. Season with salt and pepper.
3. Layer half of the frozen pierogi at the bottom of your crockpot, followed by half of the sliced kielbasa.
4. Spread half of the onion mixture over the kielbasa, then pour half of the sour cream mixture on top. Repeat the layers with the remaining pierogi, kielbasa, onion mixture, and sour cream mixture.
5. Sprinkle the shredded cheddar cheese over the top.
6. Cover and cook on low for 3-4 hours, or until the pierogi are tender and the casserole is heated through.
7. Garnish with fresh parsley before serving, if desired.

As you prepare this dish, let your mind wander back to family gatherings and the joy of sharing food. Remember, cooking is not just about following steps—it’s about infusing each moment with love and intention.