Create dough: Add flour and salt to a large bowl and mix. Add water and oil to the bowl and gently mix until a dough forms. Turn dough onto a floured surface and knead for 20-30 seconds until a smooth dough ball forms.
Roll Out Dough
Split to use: Split dough in half and keep the second half in a bowl covered by a damp paper towel until ready to use.
Prep: Prepare a large clean surface or large cutting board by lightly flouring it and keeping a small bowl of flour nearby. Take out bowls of filling and set aside with spoons or forks. Lightly flour a baking tray and set aside.
Roll out dough: Add half of the dough the floured surface and use a lightly floured rolling pin to roll out the dough 1/4 inch thick.
Cut rounds: Use a circle cutter (or glass) that's about 2 1/2 inch diameter to cut out dough circles, keeping them as close together as possible to be able to get as many circles as possible. Separate the circles from the rest of the dough, setting the excess dough aside.
Roll out and rounds: One at a time, add a dough circle to the center of your floured work station. Use a lightly floured rolling pin to roll the circle long, flip and dust with some flour on the cutting board and then roll slightly wide to create about a 3 1/2 in - 4 in oval that's about 1/8 inch thick.
Fill and fold: Add about 1 tablespoon of filling to the bottom half of the oval and fold over the top of the dough to create a half moon.Choose whatever filling combos you'd like - do all potato, all cabbage, or alternate. Pinch closed: Hold the half moon in one hand and then use the other hand to pinch it by using your thumb and first two fingers to pinch and twist the edge of the half moon closed. Do this all of the way around the half moon until it is all pinched closed. Place pierogi on floured baking sheet.Important: Be sure that it is completely closed so that when they boil, water will not get into the pierogi. Repeat: Repeat with the rest of the cut out rounds, then roll out the set aside dough and cut out more rounds and repeat until you have used all of the dough. You will fill multiple baking trays.