To make the filling:

• Wash and peel the potatoes.
• Cover with water in a medium pot. Bring to a boil over high heat. Lower the temperature and simmer until potatoes are soft, about 15 min.
• Drain the water and mash the potatoes when still hot, then leave them to cool completely. It’s important for the potatoes to not be warm when you proceed to mix the filling.
• Mince one small onion. Place 2 tbsp butter in a small pan, add the onion and saute over medium-low heat for 3-4 minutes until soft and translucent.
• When potatoes are cooled add sauteed onion, cheese, and black pepper to taste and mix well. Taste and add more salt if needed.

To make the dough:

• Place 2 cups of flour in a large bowl or on a work surface and make a well in the center.
• Add one egg to it, then add 1/3 teaspoon of salt and whisk with a fork to break the yolk.
• Add 1 cup of water, a little at a time. Continue to whisk with a fork to mix the flour with water and egg.
• Then proceed to knead the dough with your hands until smooth. You may need to add more flour, but keep in mind the dough for pierogi is much softer than for pasta.
• Let the dough rest for 15 min.

Assemble and Cook:

• On a floured surface roll out the dough to 1/8 inch in thickness. Using a 2-inch round cookie cutter or drinking glass, make small circles. (I divided the dough into 4 parts.)
• Spoon 1 heaped teaspoon of the filling onto one half of each circle.
• Fold the dough in half to form a half-circle shape and then firmly pinch the edges together.
• Sprinkle a baking sheet with flour and place the filled pierogi on it in a single layer. Cover them with a clean kitchen towel to prevent them from drying.
• To cook, bring a large pot of salted water to a boil. Drop in about 6 to 10 pierogi at a time, making sure not to overcrowd the pot. Stir once with a big spoon to make sure they are not stuck to the bottom. Cook until they rise to the surface, then cook for another 2 to 3 minutes.
• When ready, remove the pierogies from the water with a slotted spoon and place them in a large bowl. 
• Add 1 tbsp butter and shake. This will prevent the pierogi from sticking to each other.
• Serve warm with caramelized onions and a dollop of sour cream.

Caramelized onions:

• Cut the onion in half, then slice it into half circles about 1/8 inch thick.
• Place a pan on medium heat, add 2 tbsp butter and onion.
• Spread the onions out evenly over the pan and let cook, stirring occasionally.
• As soon as the onions start sticking to the pan, let them stick a little and brown, but then stir them before they burn. After 10 minutes, sprinkle some salt over the onions.
• Cook for about 15 min in total to make them brown and reach in taste.