To make Jewish potato knishes, mash potatoes with egg, parsley, salt and pepper. Add onions and oil, and mix well; set aside. Make dough into 4 balls. In 1/2 hour, take the balls and roll into a thin rectangles. Add flour, and let rest for 15 minutes. Pull dough thin, and spread one-quarter of the potato mixture in center of dough. Roll into a jelly roll, then cut 2-inch knishes. Bake at 325 degrees Fahrenheit for 25 to 30 minutes.
Bring to a boil 2 1/2 pounds of peeled potatoes in large pot of salted water, then reduce heat, and cook until soft. Meanwhile, cook 4 sliced onions in oil in covered skillet until they sweat for 20 minutes. Remove, cover, and fry until golden brown. Set aside.
Mash potatoes. Add 1 egg, parsley, salt, pepper and the onions with oil. Mix with hands, and set aside.
Make dough by beating 2 large eggs, after setting aside 1 tablespoon for glazing, and mix with oil, water, vinegar and salt. Slowly add 4 cups of flour. Start beating with spoon, then knead dough with hands. Add flour until the dough is smooth. Make 4 balls, and cover with a cloth. Let the dough rest for 30 minutes.
Roll the dough balls into thin rectangles. Add flour, and place between two wax sheets. Let the dough rest for 15 minutes. Pull dough to about 12 inches by 4 inches, and spread one-quarter of the potato mixture in center of dough, but leave a 1-inch border.
Roll the dough like a jelly roll, then cut 2-inch knishes. Place knishes flat side down 2 inches apart on a greased baking sheet. Bake in a preheated 325 degrees Fahrenheit oven for 25 to 30 minutes, or until golden brown.