Boil potatoes - Put potato and 1 tbsp salt in a large saucepan. Add cold tap water so it's 3cm/1 inch 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.

Mash - Drain, then pass the potatoes through a potato ricer into a bowl (or use potato masher)

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

Cool - Spread out ~1cm / 1/2" thick on a tray. Cover with cling wrap, pressing so it is fully in contact. Cool on the counter (~30 min) then refrigerate until cold (1 hour+).

Melt butter - Heat up the water and butter in a small saucepan over medium heat just until butter is melted, do not boil water. (Or do this in the microwave). Turn off heat.

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

Knead - Scrape out onto a lightly floured work surface. Knead for 5 minutes until the dough becomes smooth (Note 3). You could also use your stand mixer.

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

Roll & cut - On a lightly floured surface, roll out the dough to 3mm / 1/8" thickness. Cut out rounds using a 7.5 cm / 3" cutter. Do as many as you can, then gather surplus dough into a ball, wrap with cling wrap and set aside for later.

Fill & wrap - Place 1 tablespoon of cheesy potato filling in the middle of a round. Dip finger with water and run along edge of half the circle. Fold dough over to enclose the filling and press to seal, making slight indents (no pleats).

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

In a non stick skillet, melt the butter until foamy over medium heat. Add onion and salt, then cook, stirring regularly, for 10 to 15 minutes until golden on the edges. Scrape out into a bowl, then set aside (OK to cool).

Batch cooking - Directions below are for cooking and serving 10. Boil pierogis in batches of 10 to 12 max, they need space to bounce around in the water, and you need space to toss them in the butter. (Note 4 for big batch cooking)

Boil 5 minutes - Bring ~3 litres/quarts of water to the boil with 1 tbsp salt. Lower 10 piergosi into the water. Cook for 5 minutes - they should be floating on the surface.

Reserve water & drain - Dip a jug into the water and scoop out ~1 cup water. Then use a slotted spoon to transfer pierogi into a bowl (or drain, if not cooking more).

Butter sauce - Melt butter in a large skillet over medium heat. Add 1/3 of the onion butter plus the cooked pierogis with 2 tablespoons of the reserved cooking water. Toss for 1 minute, still on the stove, until the pierogis are coated in the butter. (Note 4 on scaling up)

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