Filling:
Put cut/sliced potatoes and 1 tbsp salt into a large sauce pan. Add cold tap water so it's 3 cm above potatoes. Bring to a boil on high heat, then reduce heat to medium high and simmer for 15 minutes or until potatoes are soft

Drain, then press the potatoes into a mash

Immediately add butter, cheese, salt and pepper. Mix with a wooden spoon until combined fully

Spread out 1cm thick on a tray. Cover with cling wrap, pressing so it is fully in contact. Cool on the counter (-30 minutes), then refrigerate until cold (1 hr +)

Pierogi Dough:
Heat up the water and butter in a small saucepan over medium heat just until butter is melted. Do not boil water. (Or in a microwave). Turn off heat.

Whisk the flour and salt in a mixing bowl. Make a well in the center and then add the butter water plus egg. Mix to combine into a shaggy dough

Scrape out into a lightly floured work surface. Knead for 5 minutes until the dough becomes smooth. You could also use your stand mixer

Wrap with cling wrap and leave on the counter for 30 minutes

Wrapping Pierogis:
On a lightly floured surface, roll out the dough to 3mm thickness. Cut out rounds using a 7.5cm cutter. Do as many as you can, then gather surplus dough into a ball, wrap with cling wrap, and set aside for later

Place 1 tbsp of cheesy potato filling in the middle of a round. Dip finger with water and run along edges of half the circle. Fold dough over to close the filling and press to seal, making slight indents (no pleats)

Place them on a lightly floured tray and continue to wrap the remaining Piergoi (including rolling out the remainingdough). You should get -30 pierogis

Onion Butter Sauce:
In a non-stick skillet, melt butter until foamy over medium heat. Add onion and salt, then cook, stirring regularly, for 10-15 minutes until golden on the edges. Scrape into a bowl, then set aside (ok if cools)

Cooking & Serving:
Directions below are for cooking and serving 10. Boil pierogis in patch of 10 to 12 max, they need space to bounce around in the water, and you need space to toss them in the butter

Bring 3 litres of water to the boil with 1 tbsp salt. Lower 10 pierogis into the water. Cook for 5 minutes (they should be floating on the surface)

Melt butter in a large skillet over medium heat. Add ⅓ of the onion Butter plus the cooked pierogis with 1 tbsp of the reserved cooking water. Toss for 1 minute, still on the stove, until the pierogis are coated in the butter

Slide onto a serving plate, scrapping out every drop of butter. Sprinkle with parsley, add a dollop of sour cream. Eat and be happy!!