Directions

1. Mix together the flour and salt. Add the egg to the flour and combine. The dough will be quite clumpy at this stage. Work in the sour cream and soft butter until the dough comes together in a slightly rough, slightly sticky ball.
   
2. Using just your fingertips, knead and fold the dough without adding additional flour until the dough becomes less sticky but still quite moist. Wrap the dough well in plastic wrap and refrigerate for 30 to 60 minutes, or up to 48 hours.
   
3. Combine the warm mashed potato and cheese. Stir and mash until the cheese is melted and the filling is cool to the touch. Taste and adjust the seasonings with salt and pepper.
   
4. Cut the dough into thirds. Form a log and cut into 10 pieces. Roll each piece of dough into a circle about 1/8" thick. Place 1 1/2 teaspoons of filling on each round of dough. Gently fold the dough over, forming a pocket around the filling. Pinch the edges of the pierogi to seal, then seal again with the tines of a fork then place on a parchment lined sheetpan with a towel to cover. Repeat with remaining dough.
   
5. At this point the pierogi can be frozen for up to 4 weeks, or refrigerated overnight, or cooked in a large stockpot of boiling salted water. Only cook about 10 pierogi at a time, so that they have room to float without sticking. When the pierogis can ok for 5 minutes they're done. The time will vary depending on if they're fresh or frozen.
   
6. Serve as is or crisp up with some oil in a nonstick pan. Top with onion gravy and sour cream.