Combine potatoes, cheese, onion and salt and pepper to taste.
3
In a deep bowl mix flour and salt. Add in egg, oil, and water to make a medium soft dough. Knead on floured surface until dough is smooth (caution if you knead the dough too much it will be tough and hard to work with). Divide into 2 parts cover and let rest about 10 minutes.
4
Roll the dough quite thin on a floured surface (I also did it between 2 sheets of parchment) cut circles with a large biscuit cutter, place a spoonful of filling in the center leaving enough room to seal the edges
5
In a large large pot of rapidly boiling water place 3-4 Pierogies at a time, cook for 3 to 4 minutes and place on a baking sheet.
6
In a frypan place the 4 tablespoons butter and quarter cup oil and heat. When the oil and butter mixture is hot enough to fry the pierogies until they are golden brown on each side and place on a paper towel to drain.
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Sometimes I find it easier to double the dough recipe