Cook the pierogi according to the package directions. Heat the oil in a medium skillet over medium-high heat. Add the onion and cook, stirring occasionally, until soft, about 6 minutes. Add the apple and cook 1 minute. Add the cabbage, vinegar, 1/2 teaspoon salt, and 1/4 teaspoon pepper and cook, stirring, until the cabbage is slightly wilted but still crunchy, about 3 minutes. Divide the cabbage and apple among individual plates. Serve with the pierogi and sour cream on the side. Food Fact: The Polish dumplings called pierogi come with sweet and savory fillings. Though pierogi can be boiled, panfrying is the best way to release and intensify their flavors.