Making a potato knish requires 2 cups of onions, 4 cups of potatoes, 1 egg, 1 cup water and 1/2 cup vegetable oil. Other required ingredients are 4 cups of flour, 1/2 cup parsley, 1 tablespoon vinegar and 1/2 cup egg whites, plus salt and pepper to taste.
First julienne the onions and caramelize them in a skillet. Mash the potatoes, allowing them to cool. Next mix the potatoes, onions, parsley and egg in a bowl, seasoning with salt and pepper. In another mixing bowl, prepare the pastry by first combining oil, water, vinegar and salt, then adding the flour one cup at a time. Knead the dough together and separate into four equally sized balls, covering them with a cloth for 30 minutes.
Roll the first dough out thinly, sprinkle it with flour and cover it with parchment paper. Let sit an additional 15 minutes. Spread a quarter of the prepared filling inside on top, allowing a 1-inch border to remain on the sides. Gripping the parchment paper, roll the dough closed, intermittently brushing the top with vegetable oil. Form 2-inch dumplings, then repeat the whole procedure with remaining dough balls and filling.
Place the knishes on a greased baking tray and brush each with egg white. Bake the knishes at 375 degree Fahrenheit for approximately 20 minutes. If they don't appear done after 20 minutes, bake an additional 10 and recheck. Serve with extra parsley as garnish.