CARAMELIZED ONION AND POTATO KNISHES

Steps:

• For the pastry: Whisk together the flour, baking powder and salt in the bowl of a stand mixer. Add the schmaltz, vinegar and 1/2 cup water and stir into a shaggy ball with a spoon or large rubber spatula. (It's OK if some of the flour remains dry at the bottom of the bowl; the dough will come together during kneading.) Attach the dough hook to the mixer and knead on medium speed until the dough is smooth and elastic, about 8 minutes. Dump out the dough onto a lightly floured surface and knead it into a uniform ball, about 30 seconds. Wrap the dough in plastic wrap and chill for at least 30 minutes while making the filling. (The dough can be made and refrigerated up to 1 day ahead.)
• For the filling: Heat the schmaltz over medium-high heat in a medium heavy-bottom pot. Add the onions and 2 teaspoons salt and stir until coated. Cover and cook the onions, stirring occasionally, until they are translucent and begin to brown at the edges, about 15 minutes. Continue to cook, uncovered, stirring occasionally at first and then more frequently as the onions become sticky, soft and dark golden brown, about 30 minutes. Add the thyme and garlic and cook, stirring and scraping up all the browned bits from the bottom of the pan, until the mixture is very fragrant, about 1 minute. Remove from the heat and set aside.
• Cover the potatoes with cold water in a medium pot. Bring to a boil, then lower to a simmer and cook until the potatoes until tender, 15 to 20 minutes. Strain the potatoes and then return them to the pot. Cook the drained potatoes over low heat, shaking the pot to prevent sticking, until the potatoes are dry, about 1 minute.
• Combine the potatoes, caramelized onion mixture, cream cheese, sour cream, dill and chives in a large mixing bowl. Use a potato masher to mash the mixture together until everything is evenly combined. Season generously with salt and black pepper. Set aside to cool to room temperature. (The filling can be made and refrigerated in an airtight container up to 1 day ahead.)
• To form the knishes: Preheat the oven to 350 degrees F. Line 2 baking sheets with parchment or spray lightly with nonstick cooking spray and set aside.
• Beat the egg with 1 tablespoon water in a small bowl and set aside.
• Divide the dough in half. Use your hands to roll 1 of the dough halves into a 6-inch-long log on a lightly floured surface, then use a rolling pin to roll the log out into a 16-by-12-inch rectangle. The dough will be very thin. Square off the sides, if necessary to prevent the end pieces from being too thick and uneven.
• Position the dough so that the long edges are parallel with the edge of the counter. Shape half of the filling into a 2-by-16-inch log. Transfer the filling on top of the dough, 1 inch in from the bottom edge. Gently stretch the dough up and around the filling. (It's ok if the dough tears a little as at this stage as it will be hidden as the dough is rolled up.) Then continue rolling up the filling in the dough into a long log. Score the log in 2-inch intervals and then cut the log into 8 segments.
• Working with one segment at a time, turn the piece so one of the cut side is up and pinch and smooth the top until sealed. Turn over so the other cut side is facing up, and pinch and smooth that side until sealed as well. Place the formed knish, with a sealed side down, on one of the prepared baking sheets and then gently press down to flatten it slightly. Repeat the process with the remaining segments, pastry and filling until all the knishes have been formed (see Cook's Note).
• Arrange the knishes about 1 inch apart on the baking sheets. Lightly brush the tops and sides with the reserved egg wash. Bake, rotating the pans halfway through, until the knishes are golden brown, 45 to 50 minutes. Cool at least 15 minutes before serving.
• The knishes can be served hot, warm, or at room temperature with sour cream.