In a medium bowl (best if it has its own cover, otherwise you will need plastic wrap), whisk together your dry ingredients, make a well in the center of the flour mixture and add the egg, oil, vinegar, and water. Stir together till all the ingredients are incorporated and then right in the bowl, knead the dough about six times, just pressing and turning to make sure all the ingredients have been properly mixed.

Cover the bowl with the lid or plastic wrap and let the dough sit for an hour at room temperature. Set aside.

Prepare the filling by boiling the potatoes with a pinch of salt till very soft. Put potatoes in a bowl to cool. Saute the onions in the oil till medium brown and add to the potatoes and mash together and add a teaspoon of salt. Stir well. When the filling is cool, beat the egg and pour HALF the egg into the potato mixture saving the other half to brush on the finished knishes.

Divide the rested dough into 4 parts. Roll out the dough on a floured piece of parchment paper into a rectangle and along the bottom of the dough (short side) leaving a bit of a margin, place a layer of potato filling. Encase the filling with one turn of dough and then using the parchment paper, lift up the edge and gently roll and the dough should fold over itself like a jelly roll. Pinch the dough at intervals (I did 4-5 knishes from a rectangle of dough) and twist. Cut at the twist, pinch the ends shut and sit on their bottoms, gently poking the top to help seal and make a small indentation. Place on parchment paper on a rimmed baking sheet, brush with the remaining egg mixture, and do this with the rest of dough and filling. If you have filling left over, serve as a side dish. Bake the knishes 30-40 minutes or till golden brown. Serve with mustard, of course. A note, these reheat beautifully and will keep (ha! try it and see how long they last) at least 5 days in the fridge.