A fifty-fifty mixture of farmer's cheese and boiled potato, mashed together with diced onion that's been sweated in butter. Salt and pepper as necessary. I wanted to add some fresh herb, but my sweetie pie said NO!
3 cups of all purpose flour, two eggs, two tablespoons of melted butter, a pinch salt, and a cup of potato water* BE SURE TO RESERVE A CUP OF THE COOKING WATER FROM THE POTATOES.
Making the dough
Put the flour into a large mixing bowl and form a "well" in the center. (Note, incorporate the salt into the flour evenly.) Crack the eggs into the well and add the melted butter. Then add some of the potato water and start gradually incorporating some of the flour into the egg mixture. Use a fork. As slowly and evenly as possible, pull the flour into the egg mixture. Use a circular, whisking motion. Add some more water as necessary. When the dough gets too shaggy to allow for whisking, go ahead and dump the whole mass onto your dusted counter and begin kneading the dough. Knead the dough for five minutes until it is smooth. If your dough is sticky and very soft, continually dust it with flour while kneading until it begins to harden up. A soft sticky dough will be a nightmare to work with. Once your dough is properly kneaded, pop it into the fridge for at least ten minutes.
Rolling the dough and forming the pierogi
This is perhaps the trickiest part for the novice. Like I said before, if your dough is too soft or too sticky (they are one and the same thing), rolling out the dough will be a total nightmare. It is best to knead your chilled dough again and again until it is somewhat firm and not sticky on the outside, but doughy enough that the pierogi will still seal up.
OK, let's assume your dough is perfect. Go ahead and roll out the dough to the desired thickness. Chances are, you will not be skilled enough to roll out the dough too thinly; more likely you will not make it thin enough. You do not want the gnaw through half a foot of dough. So make them as thin as you can, and then, with a glass (3" diameter) cut out circles. (Save the scrap to roll out again. Wad it up and wrap it.) Plop some of your chilled filling onto the circles, about a tablespoon per plop, and then fold the dough in half, over the filling. Then pinch them closed with your fingers. Once you have formed them all, you can boil them in groups of eight or nine. When they float to the surface, boil them for another two minutes. If you are going to freeze some, boil them for a minute. Whatever you do, drain and cool. Then finish them in pan with butter and onions.