1. To prepare the Pierogi Dough, mix together the flour and salt. Beat the egg, then add all at once to the flour mixture. Add the ½ cup sour cream and the softened butter pieces and work until the dough loses most of its stickiness (about 5-7 minutes). You can use a food processor with a dough hook for this, but be careful not to overbeat. Wrap the dough in plastic and refrigerate for 20-30 minutes or overnight; the dough can be kept in the refrigerator for up to 2 days.
2. To make the Potato, Cheese & Onion Filling: Peel and boil potatoes until soft. While the potatoes are boiling, saute onion in butter until soft and translucent. Mash the potatoes with the sauted onions and grated cheddar cheese, adding salt and pepper to taste. You can also add some fresh parsley, bacon bits, or other enhancements if you desire. Let the potato mixture cool and then form into 1″ balls.
3. To prepare the Pierogies: Roll the pierogi dough on a floured board or countertop until 1/16″ thick. Cut circles of dough (2″ for small pierogies and 3-3½″ for large pierogies) with a cookie cutter or drinking glass. Place a small ball of filling (about a tablespoon) on each dough round and fold the dough over, forming a semi-circle. Press the edges together with the tines of a fork.
4. Boil the pierogies a few at a time in a large pot of water. They are done when they float to the top (about 8-10 minutes). Rinse in cool water and let dry.
5. Saute sliced onions in butter in a large pan until onions are soft. Then add pierogies and pan fry until lightly crispy.
Homemade Pierogi Tips:
If you are having a hard time getting the edges to stick together, you may have too much flour in the dough. Add a little water to help get a good seal.
If you don't want to cook all of the pierogies right away, you can refrigerate them (uncooked) for several days or freeze them for up to several months.