Mix flour and salt in a bowl
Add egg, water, and oil or melted butter
Knead until smooth
Cover and let the dough rest
Boil potatoes until soft
Mash potatoes with butter, salt, and pepper
Add cheese, sautéed onions, or other filling ingredients if desired
Roll out the dough on a floured surface
Cut the dough into circles
Place a spoonful of filling in the center of each circle
Fold the dough over the filling
Press the edges firmly to seal
Boil the perogies in salted water until they float
Remove and drain
Pan-fry in butter or oil until golden brown if desired
Serve with sour cream, onions, or bacon if desired