In a mixing bowl, combine flour and salt. Add egg and warm water slowly. Mix until a soft dough forms. Knead gently for five minutes until smooth and elastic.

Cover the dough with a clean cloth. Let it rest for at least 20 minutes. This makes it easier to roll and shape later.

In a bowl, mix mashed potatoes, cheese, butter, salt and pepper. Stir well until smooth and creamy. Taste and adjust seasoning if needed.

Roll the dough on a floured surface. Make it thin but not too thin. Use a round cutter or glass to cut circles evenly.

Place a spoon of filling in the center of each dough circle. Fold in half to form a half-moon shape. Press edges tightly to seal.

Bring a large pot of salted water to a boil. Add pierogi in batches. Cook until they float to the top. Remove with a slotted spoon.

Heat butter in a pan. Add sliced onions and cook until golden. Add boiled pierogi and fry lightly until edges are slightly crispy and golden brown.

Serve pierogi warm with extra butter or sour cream. Enjoy the soft texture and rich filling with every bite.