Place the sliced onion in the bottom of the Crock Pot. Season with ½ teaspoon salt and ¼ teaspoon black pepper.
Place the kielbasa chunks over the onions.
Add the frozen pierogi to the Crock Pot, arranging them as evenly as possible.
In a bowl, mix the chicken broth, Dijon mustard, remaining ½ teaspoon salt, remaining ¼ teaspoon black pepper, and garlic powder.
Pour the seasoned chicken broth over the kielbasa and pierogi.
Place the sliced butter over the pierogi.
In a small bowl, mix the drained and rinsed sauerkraut with the brown sugar and caraway seeds.
Spread the sauerkraut mixture over the top of the pierogi.
Cover and cook on LOW for 3 hours.
Halfway through cooking, check the pierogi. If some aren't getting enough liquid or are drying out against the sides of the Crock Pot, carefully rearrange them so they are exposed to some of the cooking liquid.
Continue cooking until the pierogi are tender and heated through.
Serve with a dollop of sour cream and enjoy!