Mix together the flour and salt. Add the egg to the flour and combine. The dough will be quite clumpy at this point.
Work in the Sour Cream and Soft Butter until the dough comes together in a slightly rough, slightly sticky ball.
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 30-60 minutes, or up to 48 hours.
To make the filling:
Combine the warm mashed potatoes and cheese. Stir and mash until the cheese is melted and the filling is cool to touch. Taste and adjust the seasonings with salt and pepper.
To fill the pierogi
Roll half the dough 1/16" thick. Use a medium cutter to cut circles of dough. Repeat with the other half of the dough. Save the scraps; these can be snipped into small pieces and added to simmering soups.
Place 2 tsp of filling on each of the rounds of dough. Use small and medium Pierogi makers to fold and seal.
At this point the pierogi can be frozen for up to 4 weeks, or refrigerate overnight, or cooked in a large stockpot of boiling salted water. Only cook about 10 pierogi at a time, so they have room to float without sticking. When the Pierogi float, they're done. The time will vary depending on if they are fresh or frozen.
Sauté the onions in the butter in a large skillet until the onion begins to brown. Add the drained pierogi and cook until browned and crisp. Serve hot with additional sour cream, applesauce, or other condiments.