• In a large bowl, combine the water, milk, butter, and egg yolks; mix until cohesive. Gradually mix in the flour and sea salt until it thickens into a dough-like consistency.
• Flour a clean flat surface, transfer the dough to it, and gently knead it for about two minutes or until it is smooth. The dough should feel somewhat like pizza dough, elastic but not wet. Add a little extra flour if the dough is too moist. Divide the dough into two, shape into a rough ball, and place them both into a lightly oiled bowl. Cover and let rest for at least 30 minutes. You can let it rest overnight if you're trying to get a head start the next day.

  While the dough rests, you can work on your filling.
  

Potato and cheese filling

• Bring a large pot of salted water to a boil. Add the potatoes (peeled) and cook for about 15 minutes or until tender and can easily be pierced knife. Drain the potatoes and mash them with a potato masher. Halfway through, add a splash of milk, the cheese, and some salt and pepper, and continue to mash until cheese is blended and potatoes are smooth. Taste, season as needed, and set aside.

Butter and onions

• Melt about 3 TBSP of butter in a medium sauce pan. Add the finely diced onions and cook until softened and translucent, about 5-7 minutes. Transfer to a large bowl or pot and set aside.

Dough

• Transfer one of the dough balls to a clean and lightly floured surface again, and with a rolling pin, roll it out and flatten it.1/8 inch thick. Use a round drinking glass or round cookie cutter, about 3 inches in diameter, to cut out your circles. Set them aside. With the leftover dough, knead it and roll it out, and repeat the cutting process until you have none left. Then do the same with the other ball of dough. (I separate the dough because it's easier to roll out/work with smaller amounts of dough.)
• Plop about 1 heaping teaspoon of potato mixture into the centre of the dough circle. Fold the circle over the potato filling and pinch the ends very tightly to ensure that the dough is sealed.
• As you go, place the finished pierogi on a large clean towel. You can cover them to prevent drying while they sit out.

Cooking the pierogi

• Bring a large pot of salted water to a boil. Place about a dozen pierogi into the water at a time. Stir to prevent sticking. When they're ready, they'll float to the top. Transfer floaters to the pot of buttery onions and toss. Then transfer to freezer bags or storing containers. (The butter keeps them lubricated and prevents sticking; the onions are just there to be delicious.) Repeat until done.
• You can eat these fresh pierogi right away, and freeze what you don't plan to eat within five days. When you want to cook from frozen, simply boil them until they float and give them a quick fry in a pan with some butter and onions; serve with sour cream and chives.