"This authentic Polish pierogi recipe is from my grandmother and has been handed down in our family for generations. The pierogi are filled with cheese, potatoes, and mushrooms but you can use the dough recipe and stuff them with other fillings to your liking. There are many ways to serve pierogi - either topped with fried onions and parsley or serve them with sour cream, melted butter, and fried pieces of bacon."
Pass farmer's cheese and cooked potatoes separately through a food grinder or food processor.
Heat oil in a large skillet over medium heat and cook mushrooms and onions until soft, about 15 minutes. Remove from heat.
Combine farmer's cheese, potatoes, mushroom mixture, sour cream, salt, and pepper in a bowl. Mix together and set filling aside.
Place flour on a clean work surface and make a well in the center. Crack both eggs into the well. Add butter and a few tablespoons of the warm water. Mix with your hands, gradually adding more warm water, 1 tablespoon at a time, as you go. Knead well, continuing to add more water as needed. Knead until dough is soft and smooth, adding more flour to the work surface as needed.
Cut off 1/4 of the dough and roll out to a thickness of 1/8 inch. Cut out circles using a glass or a round pastry cutter, saving the excess dough for your next batch.
Fill each dough circle with 1 teaspoon of filling. Fold dough over into a half-moon shape and pinch edges together to seal. Cover with a clean dish towel so pierogi won't dry out and repeat with remaining dough and filling.
Bring a large pot of salted water to a gentle boil. Add the pierogi in batches, about 20 at a time, and cook until they float to the surface, 10 to 15 minutes. Remove with a slotted spoon and drain in a strainer. Repeat with the remaining pierogi.