Make the dough: Combine all dough ingredients in a large bowl and mix with a wooden spoon until well combined. If using a stand mixer, knead with dough hook attachment until dough is soft and pliable- it should pull away from the bowl sides while kneading. If dough is too sticky, add a few tablespoons of flour. If kneading by hand, knead for 3-4 minutes on a lightly floured surface, careful to not add too much extra flour, until smooth. Place the dough in a large greased bowl and allow it to rise, covered in plastic wrap, in a warm spot for 90 minutes until light, well-risen, and almost doubled in size.  

While dough is rising, make the filling: Heat oil in a skillet and add the scallions. Saute until scallions are soft but still bright green. Salt and pepper to taste. Cool, set aside. Cut cooked, cooled potatoes into large cubes, add to a large bowl and crush lightly with a fork to break up into smaller pieces. Add scallions, dill, and egg to the potatoes and mix well to incorporate all ingredients evenly. Should be consistency of chunky mashed potatoes. Taste and re-season with salt and lots of pepper to taste. Set aside. 

Line 2 large baking sheets with parchment paper. Once dough has risen, remove from the bowl onto a lightly floured surface and divide into 4 equal pieces. Then, divide each of those into 6 pieces. Working quickly, roll each piece into a ball and place on baking sheets. Cover with a lightly dampened kitchen towel to rest for 15 mins. Working with one ball at a time, and adding flour as needed to prevent sticking, roll each out into a 31⁄2 inch circle with a flour-dusted rolling pin. Place 2 tablespoons of filling in the center of each dough round, then bring the 2 sides of the dough up and around the filling at the top of the round, pinching the dough closed around the filling as you move down the center to close up and encase the filling. For a fancier pleat, start by pinching the dough at the top to seal and create a starting point. Then, carefully pull each side up and over the filling to meet the dough on the other side, then pinch to seal as you pleat. Work your way down the center by alternating sides as you pull and pinch one side over the other to create a decorative, braided pleat. Keep these under a towel as you finish each one so they do not dry out or crack. 

When all pirozhki are pleated and sealed, cover with a light cotton towel or pillowcase and allow to rest again and rise slightly for 20-30 minutes. Preheat the oven to 375 degrees and prepare the egg wash by whisking together 2 egg yolks with 2 tablespoons of milk. 

After the final rise, uncover the pirozhki and make sure they are spaced at least an inch apart across the 2 baking sheets. Brush each one well with the egg wash and bake for 20 minutes or until they are golden all over. Cool on a rack for 5-10 minutes and serve warm or at room temperature. Serve warm.

Make Ahead: Pirozhki can be stored at room temperature for up to 1 day, and in the fridge beyond that. Remove from fridge and let come to room temp or re-warm in oven for 10-12 minutes at 300 degrees.