1. Combine all filling ingredients and set aside.
2. Combine ingredients for dough; knead until smooth. Roll out and cut into three- by three-inch squares. Place one tablespoon potato filling in the center of each square.
3. Pinch the two opposite corners of the square to form a triangle. Seal edges tightly with tines of fork.
4. Cook in boiling, salted water with one tablespoon of oil for 10 minutes. Remove with slotted spoon.
5. Sauté onions in oil until golden brown. Warm pierogen in sautéed onions.