In the bowl of a stand mixer, add the flour and make a well in the center and add the wet ingredients; the egg, oil, vinegar and water. Use a dough hook to mix together until it forms into a cohesive mass. You may need to use a spatula every so often to push down the flour from the sides.

Bring the dough onto a lightly floured surface and knead by hand for about 5-8 minutes until it becomes a smooth and slightly elastic dough.

Cover the knish dough with plastic wrap and allow the dough to rest in a bowl for about an hour, while you make the filling.

Boil the potatoes until very soft and can easily be pierced with a fork. Drain the potatoes and add to a large bowl and mash.

In a medium skillet, add olive oil and chopped onions and saute until deeply caramelized and soft, about 10-12 minutes.

Add the spinach and saute for another 2-3 minutes until the spinach is just wilted. Add mixture to the potatoes.

Mash the potatoes and spinach mixture together and mix to combine well. Season with salt and pepper and set aside to cool.

Once the dough has been rested, it will feel pretty oily which is ok. Cut the dough in half and roll out the first piece of dough to about a 9 x 13 rectangle.

Add half of the potato mixture onto the dough and form into a long rectangle log. Brush the seam of the dough with water to help it stick.

Fold the dough over twice so the dough wraps around the potato filling and place the log seam side down.

Cut log into 8 pieces, pinching the dough on one of the sides and gently forming into a little round knish.

Place the knishes onto a parchment lined baking sheet and brush with egg wash and sprinkle with sesame seeds.

Bake at 375 degrees Fahrenheit for 30-35 minutes until golden brown.

Serve the knishes as is or with a side of deli style mustard.