In a large pot, bring enough water to cover the pierogies to a boil. Place pierogies in boiling water and cook through. Remove pierogies from heat and strain. In a medium skillet, heat 2 tablespoons of butter. Add the sliced mushrooms and stir until they are coated with butter. Add the chopped onion to the skillet and saute until the onions are soft and the mushrooms begin to give up their liquid. Remove the skillet from heat and add the sour cream and stir until just combined.
In another skillet, heat the remaining 2 tablespoons of butter. Add the cooked pierogies to the skillet and saute until browned. Transfer the pierogies to a serving platter and spoon the mushroom sauce on top.