1. Separate the chard stems from the leaves. Trim off and discard the tough ends of the stems. Cut the remaining stems in half crosswise, and then thinly slice them lengthwise to make thin matchsticks. Stack the Swiss chard leaves and slice them into 1-inch-wide ribbons. Repeat with the spinach leaves.
2. Heat the olive oil in a large skillet over medium heat. Add the onion, celery ribs and leaves, and the chard stems and cook, stirring often, until the onion and celery are very soft, 5 to 8 minutes (if the onion starts to brown, reduce the heat to medium-low).
3. Stir in the tomato, garlic, lemon zest, red pepper flakes, and black pepper and cook, stirring often, until the tomatoes become juicy and are starting to break down, 2 to 3 minutes. Add half the chard leaves and cook, stirring, until they begin to wilt, then add the remaining chard leaves. Repeat with the spinach leaves, and once they wilt, stir in the lemon juice and the salt (see Note). Transfer the mixture to a medium bowl and let it cool completely (you can refrigerate the mixture for a few hours to get it extra-cold).
Note: Taste your cheese for saltiness before salting the filling. Some types of Kashkaval (like feta) are saltier than others, so keep this in mind when seasoning the sautéed vegetables. You can always add more salt, but it’s difficult to take it away.
4. Place the cooled filling in a sieve and drain off any extra liquid, lightly pressing down on the vegetables to extract even more moisture (but don’t mash them). In a small bowl, beat the egg with the water and a pinch of salt to make an egg wash. Lightly flour your work surface and set the puff pastry on top. Lightly flour the top of the pastry, and then roll it into a rectangle approximately 20 by 12 inches and 1/16 inch thick, with a long side facing you. Working quickly so the dough doesn’t warm up, divide it lengthwise into 5 equal strips (each about 4 inches wide). Brush the strips of pastry with some of the egg wash. Spoon about ½ cup of the filling down the center of each strip, and top the vegetables with a generous ¼ cup of the crumbled Kashkaval. Reserve the remaining egg wash.
5. Fold the right edge of each strip over the filling to meet the left edge, and press the edges together to seal. If the dough starts to become sticky or difficult to work with, place it on a sheet of parchment paper and chill it until it firms up. Turn each filled strip over, seam-side down. Shape each strip into a U-shaped spiral, and place them on a parchment paper–lined rimmed sheet pan (leave space between the spirals so the sides can brown). Refrigerate the burekas until they are chilled.
6. Preheat the oven to 400°F.
7. Brush the burekas with the remaining egg wash and bake until they are golden brown, 30 to 35 minutes. Remove from the oven and serve warm or at room temperature.
Excerpted from "Breaking Breads" by Uri Scheft (Artisan Books). Copyright © 2016.
For more recipes from Uri Scheft, click here.