Add the egg, water, vegetable oil, and vinegar into the bowl of a stand mixer fitted with a dough hook attachment. Mix on medium for 30 seconds to combine. Add the flour, salt, and baking powder. Mix on medium-high until a sticky smooth dough is formed and it starts pulling away from the sides, about 12-14 minutes

Transfer the dough to a lightly oiled bowl. Cover with plastic wrap or a damp towel and let it rest room temperature for 1 hour.

While the dough rests, place the potatoes in a pot and cover with cold water and ½ tablespoon of salt. Bring the water to a boil over high heat, then reduce to heat to medium and simmer until the potatoes are fork tender, about 10 minutes. Drain the potatoes, and transfer to a large bowl. Mash the potatoes until smooth using a fork or potato masher. 

Heat the schmaltz or vegetable oil in a large frying pan over medium heat. Add the onions, 1 teaspoon of kosher salt, and black pepper, and mix to combine. Saute the onions until they are softened and start to brown, stirring occasionally, about 15-20 minutes. Add the onions to the potatoes, and mix well. Taste and adjust seasoning if needed.

Preheat the oven to 400F, and line 2 baking sheets with parchment paper. 

Cover a work surface with a clean thin cotton cloth or bedsheet. Sprinkle lightly with flour and place the dough onto the surface. Using a rolling pin, roll out the dough into a large circle. Once it gets too large for the rolling pin, begin to gently stretch the dough by hand until it measures 20-inches in diameter. To avoid tearing, rest the dough every once in a while between stretching.

Place the filling evenly in a ring shape along the edge of the circle. Rub 1 teaspoon of oil into the center of the dough, and press a finger into the middle, creating a small hole.

Roll the edge of the dough over the filling, encasing it. Continue rolling into the center until the dough is a long ring log shape. As you roll, the center of the dough will tear.

Use the heel of your hand to cut 3-inch long pieces from the log to create each knish portion. Once all the knishes are separated, twist each knish end closed to seal in the filling, and stand them up vertically on the baking sheets. Use your thumb to press down the top sealed end.

Brush each knish lightly with oil and bake until the knishes are golden brown and cooked through, about 30-35 minutes.