**Optional Step – Sauté the Onion:** Heat olive oil in a skillet over medium heat. Add the diced onion and cook until softened, about 3 minutes. This step adds extra flavor but can be skipped if desired.
**Layer Ingredients:** Place frozen pierogis in the bottom of the slow cooker. Add the sliced kielbasa and sautéed onion (if using) on top.
**Mix the Sauce:** In a mixing bowl, whisk together chicken broth, cream of mushroom soup, heavy cream, garlic powder, and black pepper until smooth. Pour the mixture over the pierogis and kielbasa.
**Cook:** Cover the slow cooker and cook on low for 4–5 hours or on high for 2–3 hours. Stir gently halfway through cooking if possible.
**Add Cheese:** Sprinkle shredded cheddar cheese over the casserole during the last 30 minutes of cooking. Cover and let the cheese melt completely.
**Serve:** Garnish with fresh parsley, if desired, and serve warm.