Making dough:

1. In food processor, pulse together flour, salt, sour cream/greek yogurt, egg and butter until it is slightly sticky and tacky. Transfer onto lightly floured surface and knead until smooth. Shape into ball and wrap in clingfilm. Refrigerate until ready to use.

Making filling, construction & cooking:

1. In large bowl, mash potatoes. Set aside.
2. In large pan, break up sausage links and sauté with garlic until fully cooked. Add sauerkraut and pepper. Continue cooking for 5 minutes. Remove from heat, add mixture to mashed potatoes along with cheese. Mix until well combined.
3. Remove dough from refrigerator and place on lightly floured surface. Roll out until ¼-inch thick. Use a 4-inch round glass or pastry cutter to cut out rounds for pierogi.
4. Fill each one with 2 tablespoons filling and fold into halfmoon shape. Gently but firmly seal pierogi by pinching and squeezing edges together.
5. Bring large pot of water to a boil.  Gently add 6 pierogi to pot. Boil pierogi until they float to the top of the water. Carefully drain and place on nonstick baking sheet until ready to fry. Repeat until all 24 are cooked.
6. Heat olive oil in large pan over medium-high. Fry pierogis on each side 2 minutes, until golden and crispy. Remove and place on serving platter. Once all are cooked, add onion to pan. Saute until translucent and caramelized.

Serving:

1. Plate pierogis with sautéed onions, top with chives and dollop of sour cream.