Traditional pierogies are pasta filled with mashed potatoes and swimming in butter and caramelized onions. You can make them from scratch, but it is not necessary thanks to pre-made, frozen varieties available in the grocery store.
Meanwhile, melt the butter in a medium skillet then sauté the onions over medium heat until they are fragrant and soft. If you have time, you can even cook them until they've started to brown. That gives them a very nice, sweet flavor.
Drain the pierogies and add them to the skillet. Sauté until the pierogies are browned.
Joe likes to sauté his pierogies without boiling them first, but I think there's more of a chance of burning them that way. I like to boil them and just brown them in the skillet.