Potato Knish

Ingredients

• 2 cups all-purpose flour
• 1 tsp baking powder
• 1 tsp kosher salt, for the dough
• 1/3 cup vegetable oil
• 1 large egg
• 1 tsp white vinegar
• 1/2 cup warm water
• 2 1/2 lb russet potatoes, peeled and cut into chunks
• 1/4 cup schmaltz or vegetable oil
• 3 large yellow onions, diced
• 2 tsp kosher salt, for the filling
• 1 tsp black pepper
• 1 large egg beaten with 1 tbsp water, for the wash

Before you cook

• Peel the potatoes and cut them into 2-inch chunks.
• Dice the onions into small pieces.
• Whisk the flour, baking powder, and dough salt together in a large bowl.
• Whisk the oil, egg, vinegar, and warm water together in a second bowl.

Steps

1. Pour the wet ingredients into the flour and stir until a shaggy dough forms.
2. Turn the dough onto the counter and knead it for 5 minutes, until it is smooth and elastic.
3. Wrap the dough and let it rest on the counter for 1 hour.
4. Put the potatoes in a pot, cover them with cold salted water, and bring them to a boil.
5. Boil the potatoes for 18 to 20 minutes, until a fork goes through easily, then drain them well.
6. Heat the schmaltz in a large skillet over medium heat and add the onions.
7. Cook the onions for 25 to 30 minutes, stirring every few minutes, until they are deeply golden and sweet.
8. Mash the drained potatoes until smooth, then stir in the onions with all their fat, the filling salt, and the pepper.
9. Taste the filling and add more salt if it is flat, then let it cool until just warm.
10. Heat the oven to 375 degrees Fahrenheit and line a sheet pan with parchment.
11. Cut the rested dough in half and roll one piece on a floured counter into a rectangle about 12 by 18 inches and paper thin.
12. Spread half the filling in a log along one long edge, leaving a 1-inch border.
13. Roll the dough up over the filling into a tight cylinder and pinch the seam closed.
14. Cut the cylinder into four pieces, then pinch and twist each cut end shut and press the piece down into a squat round.
15. Set the four knishes seam side down on the sheet pan and shape the second half of the dough and filling the same way.
16. Brush every knish all over with the egg wash.
17. Bake for 40 to 45 minutes, until the tops are glossy and deep brown.
18. Let the knishes cool for 10 minutes before serving warm with deli mustard.