These can be prepared a day or two in advance and kept in the fridge until you are ready to use them.
Boil potatoes in salted water until cooked and soft, drain and cool. Meanwhile fry the onion in butter until golden. Mash potatoes, add cottage cheese, fried onions, mix well and season with salt and pepper.
This recipe makes about enough for one recipe of fillings, we made 3 lots of dough for our 3 different fillings. The rolling out of the dough is the tricky and time consuming part. You want a dough that is soft but elastic, it should be softer to the bite than pasta for example but still firm enough to hold the filling.
Measure out your flour and tip onto a clean working bench to form a volcano shape, sprinkle a pinch of salt. Make a little well in the top of the flour volcano and carefully break the egg into this. Using a fork, quickly, but gently, start mixing the egg into the flour, as you do this start slowly pouring the water and use the fork to mix the dough together. As the dough starts to come together, sticking to the fork, gently knead the dough with your hands until it is a uniform consistency — you will need to be flouring your bench as you go to prevent the dough from sticking to it. At this stage it is easier to break the dough into two halves and work with one half at a time.
Start rolling the dough out, this will require a bit of hard work as the dough needs to end up approximately 1-1.5mm thick. Try not to over work the dough as this will make it tough. Once you have a thin dough you can start cutting discs out of it, we use the top of a drinking glass as our dough cutter.
You can cut these all out in one go, just make sure each piece is floured so they don't stick to each other. Any extra bits and edges of dough that don't fit into the glass shape can be re-rolled and cut again.
To fill your pierogi dough you will need to have some trays ready (wooden chopping boards or metal baking trays with baking paper on them work well) and a cup of water. Take one disc of dough and place a teaspoon of chosen filling in the centre, dip your finger in the water cup and then wet the edge on one half of dough disc, carefully fold the disc over itself so the edges meet and firmly press the edges together so there are no areas where the filling could leak out — the wet dough will stick to the dry dough and seal the dumpling. As you fill your dumplings place them on the prepared trays making sure they don't touch each other.
Fill a large pot with water and bring to the boil and season with salt. Once the water is at a rolling boil place as many pierogi into the pot as you can without crowding. After a minute or so gently stir the pot to prevent sticking. Once the pierogi float to the top and the water returns to the boil simmer for 5 minutes. Remove with a slotted spoon into a shallow dish, rinse with cool water and then strain. Repeat until all dumplings are cooked. We use a different pot for each filling.
Serve warm with melted butter on top.