Combine all the dough ingredients in a bowl and mix until it is smooth. If it is too dry add water, a table spoon at a time. If it is too sticky add flour. again 1 table spoon at a time.
When the dough has a smooth texture turn on to a floured work surface and kneed for 3 or 4 minutes until it is elastic. Cover and refrigerate for 30 minutes.
Sauté the onions in the butter on medium until they soften. Add the chopped mushrooms and sauté until they brown and caramelize. Add a pinch of salt and pepper to taste.
Put a large pan of water on to boil.
Roll out the 1/2 the dough on a flowered surface to about 1/4 inch thick.
Cut the dough in to circles with a pastry cutter or glass. Fill dough with mushroom mixture, about a table spoon per a pierogie and use your finger to run a bit of water around half the circle and fold shut then crimp with your fingers.
Roll out the other half of the dough and repeat.
Put the pierogies is a pan of boiling water and stir once or twice so that they don't stick to the bottom of the pan or each other. Turn the heat down to a simmer.
After a few minutes the pierogies should float to the surface. I normally let them cook for another minute to two then fish them out and drain in a colander.
Fry pierogies in (lots of) butter until crisp and browned.
