04Smash the boiled potatoes in a large bowl and add in the crumbled bacon, cheddar cheese, and the caramelized onions. Add salt and pepper, and taste test the mixture. Add more salt and pepper, if necessary.05Roll the filling into 1" balls.06Remove the dough from the refrigerator, and roll out on a floured surface to about 1/8 inch thick. Using a circular cookie cutter or the top of a glass, cut the dough into circles.07Place one ball of filling on each cut out and fold the dough into a semi circle around it. Using the tines of a fork, press together the edges of the dough to seal it together.08Bring a large pot of water to a boil and add the pierogi. Boil them until they float to the top. Remove them from the water with a slotted spoon and let dry.09Heat the remaining onion/bacon juices in the original pan. Add butter if necessary. Add a few pierogi to the pan at a time and fry until crisp.10Serve with sour cream.