Traditional Potato Knish

Instructions

1. Prepare the dough
   In a large bowl, mix flour, baking powder, and salt. Make a well in the center and add oil, beaten egg, vinegar, and warm water. Mix until a dough forms, then knead for 5 minutes until smooth and elastic. Cover with plastic wrap and let rest for 30 minutes.
2. Make the filling
   Place potatoes in a large pot, cover with cold water, add 1 teaspoon salt, and bring to a boil. Cook until fork-tender, about 15-20 minutes. Drain well and transfer to a large bowl.
3. Sauté the onions
   While potatoes are cooking, heat schmaltz or oil in a skillet over medium heat. Add diced onions and cook until golden brown and caramelized, about 10-12 minutes, stirring occasionally.
4. Finish the filling
   Mash the potatoes with a potato masher until smooth. Add the caramelized onions, salt, and pepper. Mix well and let the filling cool completely.
5. Roll out the dough
   On a floured surface, roll the dough into a large, thin rectangle, approximately 1/8 inch thick. The dough should be thin enough to see through slightly.
6. Assemble the knishes
   Spread the potato filling in a 2-inch wide strip along the long edge of the dough, leaving a 1-inch border. Roll the dough over the filling to create a long log. Pinch the seam to seal. Gently pinch the log at 3-inch intervals and twist to create individual knishes, then cut at the pinched areas.
7. Shape and bake
   Place the knishes seam-side down on a parchment-lined baking sheet. Press down slightly to flatten. Brush with egg wash. Poke a small hole in the top of each knish with a fork to allow steam to escape. Bake at 375°F (190°C) for 30-35 minutes until golden brown.