Place <quantity>500<unit>g</unit></quantity> of all-purpose flour on a work surface. Make a well in the center

Add <quantity>4<unit>tbsp</unit></quantity> of vegetable oil

Add <quantity>1<unit>tsp</unit></quantity> of salt

Crack <quantity>1</quantity> egg. Separate egg yolk and add to flour mixture

Slowly add <quantity>250<unit>ml</unit></quantity> of warm water while stirring with a spoon to incorporate

Knead dough until homogenous

Transfer to a bowl and cover with cloth. Rest 30m to relax gluten

Transfer dough to a plastic bag and rest for 30m to relax gluten

Peel <quantity>1<unit>kg</unit></quantity> of potatoes

Cut peeled potatoes in ½. Place in a pot with cold water. Turn heat to medium

Add <quantity>2<unit>tsp</unit></quantity> of salt. Cook until tender

Trim, peel and small dice <quantity>½<unit>cup</unit></quantity> of white onion

Add <quantity>1½<unit>tbsp</unit></quantity> of vegetable oil to pan on medium heat

Add <quantity>1<unit>tbsp</unit></quantity> of butter

Add diced onion. Spread out evenly

Add salt to taste

Fry until golden brown

Strain potatoes. Mash with potato ricer in a bowl

Pass <quantity>300<unit>g</unit></quantity> of quark cheese through ricer

Add all but <quantity>1<unit>tbsp</unit></quantity> of the caramelized onions

Add salt and pepper to taste

Mix until combined

Dust work surface with all-purpose flour. Cut pierogi dough in half

Roll out pierogi dough to <quantity>4<unit>mm</unit></quantity>

Cut dough into rounds of <quantity>7<unit>cm</unit></quantity> in diameter

Add <quantity>1<unit>tbsp</unit></quantity> of filling to pierogi dough rounds. Fold in half and press sides together to seal

Boil <quantity>1.3<unit>L</unit></quantity> of water in a pan. Add <quantity>1<unit>tbsp</unit></quantity> of vegetable oil and salt to taste

Cook pierogi in small batches

Remove pierogi from water when they float

Season with <quantity>1<unit>tbsp</unit></quantity> of vegetable oil

Transfer pierogi to serving plate. Garnish with <quantity>1<unit>tbsp</unit></quantity> of caramelized onions, <quantity>2<unit>tbsp</unit></quantity> sour cream and <quantity>1<unit>tbsp</unit></quantity> of chopped scallions

The Pierogi are ready! Enjoy!