- In a large bowl, combine flour and salt. In a separate bowl, whisk together egg, sour cream, and butter. Mix wet into dry until a shaggy dough forms. 
- Turn the dough onto a floured surface and knead for about 8–10 minutes until smooth. Wrap in plastic and let it rest for 30 minutes. 
- Heat oil in a skillet. Add diced onion and sauté until golden, about 4–5 minutes. 
- In a bowl, combine mashed potatoes, sauerkraut, and sautéed onions. Season with salt and pepper. Set aside. 
- Divide dough in half and roll out to about 1/8-inch thick. Cut into 3-inch rounds using a cookie cutter or glass. 
- Place a heaping teaspoon of filling into the center of each round. Fold over and seal edges with a fork. 
- Bring a large pot of salted water to a boil. Cook pierogi in batches until they float, about 3–4 minutes. Drain. 
- Optional: Pan-fry boiled pierogi in a bit of butter or oil until golden and crispy on both sides. 
- Serve warm with sour cream and a sprinkle of fresh chives.