The recipe calls for pan-frying the pierogi in butter, but we had some leftover bacon fat from breakfast and… it’s worth it. You’re frying them up anyways, might as well use the best fat lying around. If one feels the need to justify, the bacon brings out the Dijon and gruyere very nicely.
In a large bowl, mix all ingredients together until just combined. Turn dough out onto a lightly floured surface and knead a few times until dough is pulled together. Cover with plastic wrap and allow to rest at room temperature while one mixes the filling.
Start to boil a large pot of water. While water is coming to a boil, brown onions in a pan with ½ tablespoon of butter. Continue to brown until onions are completely caramelized. Remove from heat. When water has come to a boil, add potatoes, skins on, and boil for 15-20 minutes or until soft when pricked with knife. Drain, and then peel as soon as possible.
Mash potatoes in a large bowl with a fork or potato ricer. Mix in the caramelized onions, sour cream, Dijon, 2 ½ tablespoons butter, and Gruyere. Salt and pepper to taste.
Turn dough out onto a lightly floured surface. Knead a couple of times and then roll out until 1/8 of an inch thick. Cut into 20-25 round using a 3 ½ inch biscuit cutter or drinking glass. Place rounds on a floured baking pan. Scoop a flat tablespoon of filling onto half of each round and brush water on half of each round. Fold round over, sealing the edges and making sure no filling escapes or oozes out. This is very similar to gyoza making, but requires less finesse as crimping is optional.
Boil pierogi in boiling water for 5 minutes each. At this point, the pierogi can be stored in the refrigerator for three days or at room temperature for a couple of hours in a shallows baking dish and well coated in vegetable oil. Brown pierogi in butter or bacon fat before serving. Salt and pepper to taste and sprinkle with parsley. Serve hot.