Preheat the oven to 375 degrees. In a mixing bowl, combine the onions, potatoes, egg, and parsley together. Season with salt and pepper. Set aside. For the pastry: In a mixing bowl, combine the oil, water, vinegar, and salt. Add the flour, 1 cup at a time and knead into a ball. Shape into 4 balls and let rest, covered with a cloth, about 30 minutes. Roll each ball of dough out as thin as possible into a flat rectangle. Sprinkle the dough with flour and cover with parchment paper. Let the dough rest for 15 minutes. Using your hands, spread the dough out very thin. Spread 1/4 of the filling onto one third of the dough, leaving a one inch border. Holding the parchment paper, roll up the dough like a jelly roll, brushing oil across the top a couple of times as you roll. Using the side of your hand like a knife, divided the roll into 2-inch knishes. pinch the open ends shut. Repeat procedure with remaining balls and dough. Place the knishes, flat side down, 2 inches apart, on a greased baking sheet. The knishes will need to be baked in batches. Brush the knishes with the egg wash. Bake for 25 to 30 minutes or until the pastries are golden brown. Place the knishes on a platter and garnish with Essence and parsley.
This recipe is adapted from the book by Joan Nathan, Jewish Cooking in America.