CHICKEN SOUP KNISHES WITH CREAMY GRAVY

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: Combine the chicken thighs, chicken broth, garlic, carrots, onion, parsnip, bay leaf, half the parsley, half the thyme and 2 teaspoons salt in a wide shallow pot and bring to a simmer over medium-high heat. Cover the pot, and simmer gently over low heat until the chicken is very soft and tender, about 1 hour 30 minutes.
• Transfer the chicken to a plate and set aside until cool enough to handle, about 10 minutes. Use a slotted spoon to remove the vegetables to a second large plate. Place a strainer over a large bowl and strain the broth; reserve for the gravy. Transfer the strainer over a plate to catch any drips and set the vegetables and the broth aside. Wipe out the pot and set it back on the stove off the heat.
• Transfer the vegetables to a large bowl and discard the bay leaf. Mash the vegetables with the back of a fork. Pick the chicken meat, discarding the skin and bones. Chop the chicken meat finely and add to the vegetable mixture. Stir in the breadcrumbs and the remaining parsley and thyme. Season generously with salt and pepper. (The filling can be made up to 1 day ahead, but the knishes need to be formed and baked on the same day.)
• 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 the pans aside.
• Beat the egg with one 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 sides 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 10 minutes before serving. The knishes can be served hot, warm or at room temperature.
• For the gravy: While the knishes are baking, melt the schmaltz in a large saucepan over medium-high heat. Add the flour and cook, whisking, until the flour smells toasty but has no color, about 1 minute. Whisk in the reserved chicken broth and cook, whisking constantly and being sure to get the flour in the corners and sides of the pan, until the mixture begins to thicken, about 1 minute. Simmer the sauce over medium heat, whisking occasionally and running the whisk around the sides and into the corners of the pan to prevent clumping, until the sauce is thick enough to coat the back of a spoon, about 10 minutes. Season lightly with salt, if needed. Serve the warm gravy with the knishes.