To prepare the pierogi dough, sift the flour onto a large wooden board or work surface. Make a well in the centre and add the beaten egg and oil along with a few tablespoonfuls of warm water. Using a knife, begin to mix together, adding a little more water one tablespoonful at a time. At first, the dough will be quite soft and sticky. Use your hands together to bring the dough together into a ball.
Once the dough has come together, knead it on a floured surface for 4-5 minutes. The dough should become quite elastic. If it is too wet, add a little more flour. Put the dough into a bowl, cover with a damp tea towel and set aside for 30 minutes.
Put the potatoes into a large pan of cold water, add a pinch of salt and bring the water to the boil over a high heat. Turn the heat down and simmer gently for 15 minutes or until the potatoes are soft when pierced with a knife. Drain and leave to dry out completely.
Meanwhile, heat the oil and butter in a large frying pan. Cook the onion over a low heat for at least 10 minutes or until completely soft and caramelised. Leave to cool slightly.
Mash the potatoes, add the curd cheese and mash together.
Add the onion to the mashed potato mixture and season well with salt and pepper. Leave to cool completely before filling the pierogi. You can make the filling up to 2 days in advance.
Divide the dough into half and keep one half covered with a damp tea towel to prevent it from drying out. Sprinkle your work surface with flour and roll out the dough until it is about 3 mm thick.
Have a floured tray or board to hand. Using a pastry cutter or an inverted glass tumbler, cut out 8cm/3 inch circles of dough. Continue until all the dough is used. Cover the circles with a damp tea towel until you are ready to start filling - or cut out a few circles at a time and fill them as you go along, keeping the dough covered with a clean tea towel.
To fill, place a circle of dough in the palm of your hand and add a teaspoonful of filling in the centre of the circle. Fold the dough over to enclose the filling. Using your thumb and forefinger, pinch the dough along the edge so that the pierogi is well sealed. Lay the pierogi in rows on the floured tray and cover with a damp tea towel while you prepare the rest.
To cook: bring a large pan of water to the boil. Carefully drop the dumplings in one at a time (you can cook about eight in a standard pan). Keep the water at a gentle boil. The pierogi are cooked when they float to the top, after about 2-3 minutes. Lift them out with a slotted spoon, drain in a colander and set aside while you cook the rest. You can serve the pierogi boiled, as they are, or you can gently fry the boiled pierogi with a little vegetable oil or butter so that they pick up a little golden colour.
Notes
Recipe reproduced with permission from Ren Behan/Pavilion Books.