Make the Dough

Combine the flour, baking powder, and salt in a large bowl and mix well.

In a separate bowl, mix all wet ingredients (vegetable oil, egg, white vinegar
and warm water).

Make a well in the dry ingredients and pour liquid ingredients into the center. Stir to combine until a ball of dough forms. Turn dough out onto a lightly floured surface and knead until soft and elastic. Wrap it and set aside while you make the filling.

Make the Filling

Put quartered potatoes into a large pot with the teaspoon of salt, cover with cold water, and bring to a boil over high heat. Reduce heat to medium and cook until potatoes can be pierced easily with a knife, about 10-15 minutes. Drain, then transfer potatoes to a large bowl and mash until smooth.

In a large skillet over medium heat, combine butter and olive oil. When butter has melted, add onion and season with salt and pepper. Reduce heat to medium-low and cook, stirring occasionally, until onions are caramelized and golden, about 10-15 minutes. At the end of sautéing, add the thyme leaves.

Transfer caramelized onion to bowl with mashed potatoes, and stir to combine. Taste for seasoning and add more salt and/or pepper if needed.

Assemble the Knishes

Line a large baking sheet with parchment paper and preheat oven to 375° F.

Split dough in half, placing one half back under plastic wrap. On a lightly floured surface, roll out the other dough half into a large rectangle about 12" long and ⅛" thick.

Place half of the filling over one long end, about 2" away from the edges.

Beat egg yolk with water and brush over the opposite long end and a little on the sides. Roll the dough around the filling and into a log.

Use the side of your hand to create dents in the log every 2". Use a knife or bench scraper to cut the log at these points, making 6 knishes with each roll.

Set the knish onto prepared pan, with one cut end facing down and the other facing up. Form a plump round shape by squishing down the tops a bit and tucking everything into the center. Twist the upper end into a swirl with your fingers. Repeat with the remaining dough and filling.

Bake the Knish

Arrange knishes on prepared baking sheet so that they donʼt touch and brush with egg wash.

Bake, turning pan halfway through, until golden, about 30-35 minutes. Let cool at least half an hour before serving. Eat them warm, at room temperature, or cold.