In a bowl, mix together the flour, sour cream, egg, olive oil and salt until the ingredients bind together. Place the dough on a lightly floured surface and knead for a couple minutes, or until smooth. Wrap in plastic wrap and let rest for 30 minutes.
Filling & Finishing
baking potatoes, peeled and cut into 1-inch pieces
cooked kielbasa sausage, diced
Kosher or sea salt, to taste
Fresh cracked black pepper, to taste
medium onion, sliced
Place the potatoes and 1/2 teaspoon salt in a large pot and fill with cold water to cover the potatoes. Bring the water to a boil over high heat, then reduce heat to medium-low and then simmer for 10-15 minutes or until the potatoes are tender when pierced with a fork. Drain potatoes and mash.
Combine the mashed potatoes, sausage, and season with salt and pepper. Allow to cool.
On a lightly floured surface, roll out the dough to about 1/8-inch thick. Using a 3-inch round cutter, cut out 12-15 rounds. Brush the edges of each round lightly with water, then place 1 tablespoon of filling on one side of the rounds. Fold the dough over, making half moon shapes, pressing out the air. Press and crimp the edges to seal the pierogi.
Bring a large pot of salted water to a rolling boil. Cook the pierogi for 3-5 minutes, or until tender. Drain.
Heat the butter in a large skillet over medium heat. Add the onions and cook for 2-3 minutes or until they start to brown. Season with salt and pepper, then add the pierogi to the skillet with the onions and sear each side for about 1 minute or until golden. Serve warm with the onions.