Sift the flour into a bowl and add the salt.

Whisk the sour cream, butter, oil, eggs, and egg yolk in another bowl.

Now mix the sour cream mixture into the flour until well combined.

Cover the bowl with a towel and leave it alone for 20 minutes.

Put the potatoes in a pot and cover with water.

Bring to a boil and cook for 15 minutes or until tender.

Drain and mash with the cheese sauce and shredded cheese while hot.

Add salt and black pepper and onion salt to taste.

Let the mashed potato mixture cool.

Shape the dough into two equal sized balls.

Roll them out on a floured worktop until thin.

Cut it into rounds with a pierogi cutter or cookie cutter.

Brush water around the edge of each one and spoon in some filling.

Fold each one over and press with your fingers or crimp with a fork to seal the edges.

Arrange them on a cookie sheet and freeze.

Transfer them into freezer-safe bags.

When you’re ready to cook the pierogis, bring a pot of water to a boil and add a pinch of salt.

Add the pierogis one at a time.

They’re done when they float on top of the water.

Remove with a slotted metal spoon and continue until you’ve cooked the whole batch.