In a large bowl mix the flour and salt. Add the egg and stir to combine. (it won’t be much of a dough at this stage.)
Work in the softened butter and sour cream until the dough comes together in a rough, but slightly sticky ball.
Using just your fingertips, knead the dough until it is smooth, less sticky, but still and quite moist. Form the dough into a round ball and cover in plastic wrap. Refrigerate for 30-60 minutes.
While the dough is chilling combine the warm mashed potatoes, cheddar cheese, salt, black pepper, and onion powder in a medium size bowl. Stir and mash until the cheese is melted and the filling is evenly combined.
Fill the Pierogi
Roll half of the dough into 1/8” thickness. Using a 2-3 inch round cutter, cut circles out of the dough. Repeat with the other half of the dough.
Place 2-3 teaspoons of filling on each round of dough (more or less depending on the size of circle cutter you used.) Lightly wet your finger with water and wet the edge of half of the dough. Fold the dough over to create a pocket around the filling. Pinch the edges to seal, and then use the tines of a fork to create a stronger seal.
You have three options here: you can freeze the perogi (for up to 4 weeks), refrigerate overnight, or cook them right away.
Cook the Pierogi
Bring a large pot of water to boil. Once boiling add 1-2 tablespoons of salt to the water. Reduce heat to a simmer.
Place 6-8 pierogi in the simmering water. Once the pierogi float to the top, they are done.
Place the pierogi on a lightly buttered plate (so that they don’t stick).
In a skillet over medium to medium-low heat melt a few tablespoons of butter. Transfer the drained pierogi to the skillet and cook until browned and the edges crisped. Serve hot with a dollop of sour cream and a sprinkling of fresh chives.
When making the dough, it will take a few minutes of kneading for the dough to come together. At first, it's quite crumbly, but keep working and it will be perfect!This recipe is perfect if you have leftover mashed potatoes. When I make these I like to use the pre-packaged Idahoan mashed potatoes. Makes things super easy and simple!If using homemade mashed potatoes, you might want to add more than 1/4 tsp of salt in the mashed potato/cheese mixture. Taste as you go! More salt, more pepper, do it to your liking!When boiling the pierogies remember to stir the pierogies so that they don't stick together (or the bottom of the pot).