Remove the dough from the fridge and generously flour both your work surface and the dough. Roll the dough into a very thin square, running your hands occasionally under the dough to allow it to rest and contract before continuing to roll. If the dough rips a tiny bit, don’t worry, it’ll be fine. Once your square is about 23 in (58 cm) long on each side, use a flat knife or pizza cutter to trim a little less than 1/2 in (1 cm) from each side of the square to create tidy ends and divide it into three long rectangles. Divide the filling evenly between each rectangle, laying it down the middle of each rectangle, forming it into a fat sausage of filling, leaving 1 or 2 in (2.5 to 5 cm) of dough unfilled at the end of each rectangle. Then, like you’re folding a burrito, fold up the short uncovered ends of the dough, cover the filling on each dough rectangle, and fold each long side of dough up and over the filling, overlapping by about 1 inch (2.5 cm). Turn the roll seam-side down on your work surface, then slice the roll at eight even intervals — these are your knishes. Pick up a knish and, on one of the open sides, pinch together the dough to seal — this is your bottom. Place it sealed-side down on a baking sheet lined with parchment paper, about 1 in (2.5 cm) apart, then press down a bit and shape into a fat round with a little open window of potato filling. Repeat for the rest of the knishes, making two pans of nine. Brush with egg wash, then bake for 30 minutes, or until golden brown.