Combine flour, eggs, 1/2 teaspoon salt and water in a mixing bowl.

Mix well to make a medium soft dough.

Knead well until dough is firm.

Cover with a warm towel and let rest 10 minutes.

Prepare potato filling by mashing potatoes.

Sauté chopped onion in butter until softened.

Mix sautéed onion, remaining salt, and white pepper into mashed potatoes.

Roll out dough and cut into circles.

Place a spoonful of potato filling in the center of each circle.

Fold the dough over and seal the edges to form pierogi.

Boil pierogi in salted water until they float to the surface.

Sauté boiled pierogi in butter until golden brown.

Serve hot with sour cream (optional).