Begin by sifting the flour and salt into a large mixing bowl, ensuring a light and airy base for your dough. Using a spoon, gently push the egg yolk through a fine sieve into the flour for a smooth incorporation. Add the raw egg to the mixture, stirring well to combine the ingredients thoroughly. Gradually beat in the butter, one tablespoon at a time, allowing each addition to meld smoothly into the flour blend. Transfer your dough onto a floured surface and knead diligently until it becomes smooth and elastic—this texture will ensure a tender yet resilient pastry. Wrap the dough snugly in waxed paper and refrigerate for at least 30 minutes until it firms up, which makes it easier to handle later. While the dough chills, melt 2 tablespoons of butter in a heavy skillet over medium heat. Sauté the onion and rutabaga until the onion softens and turns transparent, approximately 5 minutes, releasing their natural sweetness. Next, process the sautéed vegetables and beef twice through a meat grinder if available, or finely chop them with a knife to achieve a uniform texture essential for a consistent filling. Melt the remaining 4 tablespoons butter over medium heat, adding the meat mixture. Cook gently on low heat, stirring occasionally, until the liquid evaporates and the filling thickens enough to hold its shape, ensuring a rich, concentrated flavor. Remove the skillet from heat and let the filling cool completely. Once cooled, beat in one egg to bind the mixture, and season with salt and pepper to taste for balanced seasoning. Preheat your oven to 350°F (175°C) in preparation for baking your pasties. Roll the dough out on a lightly floured surface into a thin 13x8-inch rectangle, about 1/8 inch thick, providing the perfect canvas for the filling. Spoon the savory filling evenly down the center lengthwise, leaving about an inch of space at each end to seal the pastry securely. Lightly brush the long edges with cold water to act as glue, then carefully fold one long side over the filling, followed by the other, enveloping the mixture thoroughly. Brush the short ends with cold water as well and fold them over to completely enclose the filling, creating a neat loaf shape. Place the pastry seam side down on a baking sheet lined with parchment paper. Brush the top generously with the remaining beaten egg to achieve a beautiful golden shine. Bake in the preheated oven for about 30 minutes, or until the pastry turns a rich golden brown, signaling a perfectly cooked crust. Once baked, allow the paszteciki to cool slightly before slicing diagonally into 1.5-inch pieces. These bites are delightful served warm, either as an enticing appetizer or paired with soups for an authentic Polish meal experience.