- 2 and 1/2 cups (350 g) all-purpose flour
- 1 tsp. baking powder
- 1/2 tsp. fine sea salt
- 1 egg
- 1 tsp. white vinegar
- 1/2 cup neutral oil such as vegetable or canola
- 1/2 cup warm water
- 2 lbs. Russet or baking potatoes, peeled and cut into 2-inch chunks
- 3 tsp. kosher salt, divided
- 3 Tbsp. butter, oil, or schmaltz*
- 2 yellow onions, thinly sliced
- 1/2 cup scallions, sliced
- 1/2 tsp. white pepper
- 1 egg beaten with 1 Tbsp. of water
- In a large bowl, whisk together the flour, baking powder, and salt. Whisk together the egg, vinegar, oil, and water in a small bowl or large measuring cup. Add the wet ingredients to the dry ingredients and stir with a fork until a shaggy dough forms.
- Turn the dough out onto a lightly floured surface and knead until smooth, but do not overwork the dough. Cover and set aside to rest for at least 1 hour. (May be done up to 3 days in advance. If making the dough in advance, refrigerate it until needed.)
- While the dough is resting, prepare the filling. Place the pieces of potato in a large, heavy Dutch oven or saucepan and cover with cold water. Add 2 teaspoons of salt to the water. Cover and bring to a boil over high heat. When the water is boiling, partially uncover the pot and turn the heat down to medium. Cook the potatoes until they are tender, about 10 minutes, and then drain them well. When the potatoes are dry, return them to the pot and mash them with a potato masher or put them through a potato ricer. Set aside.
- While the potatoes are cooking, caramelize the onions. Heat the butter, oil, or schmaltz over medium heat in a large, deep skillet with a tight-fitting lid. Add the sliced onions and season with the remaining teaspoon of salt. Stir to coat the onions with the fat and then turn the heat down to low. Cover the onions and cook, stirring occasionally, for 10 minutes. Uncover the skillet and continue to cook the onions, stirring frequently until very brown and soft, about 15 minutes.
- Add the mashed potatoes and scallions to the skillet with the onions and stir to combine. Add the white pepper and taste, adding more salt if necessary. Allow the filing to cool to room temperature. (May be done up to 3 days in advance. If making the filling in advance, refrigerate it until needed.)
- Line two baking sheets with parchment paper. To assemble the knishes, divide the rested dough in half. Place one half of the dough on a lightly floured surface and roll out into a 12×16 rectangle with one of the long sides facing you. Arrange half of the potato mixture in a log along the bottom of the dough, about 2 inches from the edge. Pull the bottom edge of the dough over the potato mixture and roll away from you, loosely, so there are multiple layers of dough around the potato filling.
- Using a serrated knife, cut the log of dough into six even pieces. Turn each piece 90 degrees so the exposed filling is on the top and bottom. If desired, pull the edge of the dough over the bottom of the knish and pinch it closed. Place the knishes, closed side down, on one of the parchment-lined baking sheets. Repeat this process with the remaining half of the dough and the remaining potato filling.
- If time permits, chill the formed knishes, covered, for several hours, and up to overnight, prior to baking. When ready to bake, preheat the oven to 375°F. Brush the outside of the knishes with the egg wash.
- Bake knishes until the pastry is golden brown, about 40 minutes. Serve warm with spicy brown mustard.
*Note: Schmaltz, which is simply rendered chicken fat, is a traditional ingredient in eastern European Jewish cooking. You can make your own or look for it at gourmet markets or grocery stores, where it is sometimes sold frozen. You can also substitute duck fat if that is easier to find. Using schmaltz will prevent this dish from being meatless. To keep the dish vegetarian, use butter and for a dairy-free version, use oil.