Gather all ingredients.

Prepare the dough: Stir flour and salt together in a large bowl. Whisk sour cream, eggs, egg yolk, butter, and oil in a separate bowl. Stir wet ingredients into flour until well blended and dough comes together. Cover the bowl with a towel, and let stand for 15 to 20 minutes.

Meanwhile, prepare the filling: Place potatoes into a pot, and fill with enough water to cover. Bring to a boil, and cook until tender, about 15 minutes.

Drain potatoes, and mash with shredded cheese and cheese sauce while still hot. Season with onion salt, salt, and pepper. Set aside to cool.

Separate pierogi dough into two balls. Roll out one ball at a time on a lightly floured surface until it is thin enough to work with, but not too thin so that it tears, between ⅛- and ¼-inch thick. Cut into circles using a cookie cutter, pierogi cutter, or a glass, 3- to 4-inches in diameter.

Assemble the perogies: Brush a little water around the edges of the circles, and spoon some filling into the center.

Fold the circles over into half-circles, and press to seal the edges with fingers, or use a fork for crimped edges.

Place perogies on a cookie sheet, and freeze. Once frozen, transfer to freezer storage bags or containers.

To cook the perogies: Bring a large pot of lightly salted water to a boil. Lower perogies into the water in batches. Cook until they float to the top, about 5 minutes. Do not boil too long, or they will be soggy. Remove with a slotted spoon.

Serve and enjoy!