Clear a large space on the counter. Prepare: bowl of filling with a tablespoon, dough, additional flour, coffee mug and rolling pin. On a floured surface, roll the dough into a thin circle (2mm thick).
Using a coffee mug cut the circles out of the dough.
Place 1 tablespoon (or less!) of the filling in the centre of each circle of dough and fold over. Press and seal into half-moon shapes. You can use a little bit of warm water to seal the pierogi. If the dough sticks to your fingertips - put them in a bit of flour.
Put your dumplings on a floured surface, so they don’t stick to the counter. Cover your dumplings (even while making them!) with a clean kitchen cloth, so they don’t dry out.
In the meanwhile peel the onion and cut it into cubes. Fry onion on the frying pan with the addition of butter, until tender.
Cook the pierogi in a pot of boiling, salted water until they float. After floating cook them for another 3 min then remove them from the water and drain. Melt 2 tablespoons of butter in a saute pan, fry chopped onion until tender, add pierogi to the mixture and sear them for 2 to 3 minutes on each side, or until they are golden brown.