Combine 12 cups of flour and salt and mix in a Kitchen Aid type mixer for about 30 seconds. Combine eggs, 3 cups of water, and the oil at slower speed in mixer. Then add sour cream, water oil & egg mixture to flour. Mix for about 2-1/2 minutes. At this point check dough and make sure it is creamy & pliable. Dough should not be too sticky, if it is add a little flour and water. When dough reaches desired texture (silky) remove from mixer and allow to fortify on floured table (covered) for about 30 minutes.
After 30 minutes take dough and begin working it with a floured rolling pin. Roll dough out to 1″-1-1/2″ thickness. Cut this sheet into quarter sheets. Take a sheet and roll it out to 1/4″ thickness. If dough sticks sprinkle with flour. Take a cookie cutter about 4″ in diameter and cut dough into circles.
After dough circles are cut, re-roll into 1/8″ thickness. Take desired filling & place in center of circle. Lightly wet one edge with water and folder over and firmly crimp together. Place Pierogi on floured tray.
Gently place pierogi in salted water that is just under boiling point (about 190°). Cook for about 3 minutes or until pierogis float. Brown cooked pierogis in skillet with butter or margarine. Serve with butter & onion & a dollop of sour cream with chives or fresh dillweed.
Note: Some filling mixtures that work well are Potato & Cheddar, Mushroom, Potato & Swiss, Sauerkraut, etc. Make sure your filling mixture is no too watery or soupy, but firm!!