Lightly grease the insert of a 6-quart slow cooker with non-stick cooking spray or butter.
Arrange half of the frozen pierogies in a single layer at the bottom of the slow cooker. Top with half of the sliced beef kielbasa, half of the chopped onion, and half of the drained sauerkraut.
Repeat the layering process with the remaining pierogies, beef kielbasa, onion, and sauerkraut.
In a small bowl or measuring cup, whisk together the chicken broth, garlic powder, onion powder, salt, and pepper. Pour this mixture evenly over the layered ingredients.
Cover and cook on LOW for 3 to 4 hours, or until the pierogies are tender and heated through. Avoid cooking longer to prevent the pasta from becoming soggy.
Once cooked, gently stir in the sour cream, being careful not to break the pierogies.
Sprinkle the shredded cheddar cheese evenly over the top. Cover and let sit on the 'Keep Warm' setting for about 10-15 minutes, or until the cheese is fully melted and bubbly.
Garnish with fresh chopped parsley before serving hot.