Cook the pierogi according to the package directions.
Meanwhile, heat the butter in a medium skillet over medium-high heat. Add the onion and cook, stirring occasionally, until soft, 5 to 6 minutes. Add the apple and cook for 1 minute.
Add the cabbage, vinegar, ½ teaspoon salt, and ¼ teaspoon pepper and cook, stirring, until the cabbage is slightly wilted but still crunchy, 2 to 3 minutes.
Divide the cabbage mixture among plates. Serve with the pierogi and sour cream on the side if desired.
I'll definitely make this satisfying and inexpensive meal again. Will any of you be adding it to your repertoire?
P.S. Sometimes I make my own pierogi with this recipe and store them in batches in the freezer. Other times I purchase them like I did for the recipe above. They're a delicious and convenient food to have on hand!