Cook a potato knish by combining 4 cups of mashed potatoes with 2 cups of caramelized onions, 1/2 cup of chopped parsley and 1 egg and seasoning the vegetables with salt and pepper for the filling. In a separate bowl, stir 1 cup of water, 1/2 cup of vegetable oil, 1 tablespoon of white vinegar and 1/2 teaspoon of salt. Gradually add 4 cups of flour to the liquid ingredients until dough forms, and divide it into four pieces.
After the dough rests covered for 30 minutes, roll the pieces into flat circles or rectangles, sprinkle them with flour, and cover them with wax paper for 15 more minutes. Spread the dough circles out, and spoon the filling into the center. Roll up the knishes, and pinch the edges shut. After placing the dumplings on a greased baking sheet, brush the tops with 1/2 cup of egg wash, and bake them at 375 degrees Fahrenheit for 25 to 30 minutes. Sprinkle parsley and essence on the knishes before serving.
To make mashed potatoes for the filling, peel 2 1/2 pounds of russet potatoes, cut them in half, and place them in a pot of hot. After the liquid starts to boil, reduce the temperature, and simmer the potatoes for 15 minutes. After they cool, mash them.