Preparation / Directions:
Slowly cook the onions in the oil in a skillet, covered, over a low heat. Let the onions "sweat" for about 20 minutes, or until they are soft. Then remove the cover and fry over a medium heat until golden brown. Don't drain
Meanwhile peel the potatoes and cut them in half. Put them in a large pot filled with cold water and salt to taste. Bring to a boil, then turn the heat down, and cook until soft, about 15 minutes. Drain and cool for 5 minutes.
Mash the potatoes and add the egg, the parsley, salt, and pepper. Add the onions with the oil and mix well with your hands. Set aside while preparing the dough.
Beat the eggs and reserve about 1 tablespoon of egg for the glaze. Mix the rest with the oil, water, vinegar, and salt. Add the flour gradually, beating first with a spoon and eventually your hands as you knead the dough. Continue to add enough flour to make a smooth dough. Shape into 4 balls and let rest, covered with a cloth, about a half hour to relax the gluten.
Roll each ball of dough out as thin as possible into a flat rectangle. Flour well and place between 2 sheets of waxed paper. Let sit for about 15 minutes.
Using your hands carefully stretch each rectangle as thin as possible, about 12 to 14 inches long by 4 to 5 inches wide. Spread one quarter of the filling (about 0/2 cups) onto approximately one third of the dough, leaving a 1-inch border.
Holding onto the waxed 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, divide the roll into 2-inch knishes. Then pinch the open ends shut. Repeat with the remaining balls and dough. Place the knishes, flat side down, on a greased cookie sheet, leaving a 2-inch space between each. You will have to bake in batches.
Mix the reserved tablespoon of egg with a little water. Brush the tops with the egg wash and bake in a preheated 375-clegree oven for 25 to 30 minutes or until golden brow