Make the filling:
In a medium (3 qt.) saucepan, stir together the frozen berries, sugar, lemon juice, lemon zest, cinnamon, and salt. Heat over medium high until the berries start to release their juices and those juices bubble, 1 to 2 min. Reduce the heat to medium and simmer, stirring gently from time to time, until the berries release more juices and soften but still hold their shape for the most part (raspberries will probably break down, though, and blackberries might too), 7 to 9 min. Remove from the heat.
With a slotted spoon, scoop out the berries, letting as much juice as possible drain through the spoon, and put them into a small heat-proof bowl.
Dissolve the cornstarch in 3 Tbs. cold water. Whisk into the juices in the pan. Bring to a boil over medium-high heat. Boil, stirring with a wooden spoon, until the mixture is very thick, a full 2 minutes. Scrape the sauce into the bowl with the berries. Fold them together. Taste. If too tart, add a little more sugar; if too sweet, add a little more lemon juice. Let the filling cool completely before using.
Assemble the jalousie:
Line a large rimmed baking sheet with parchment. In a small bowl, make an egg wash by beating the egg with 1 Tbs. water until well combined.
Unfold the puff pastry dough on a floured surface, and gently pinch together any seams that have split. With a floured rolling pin, roll the dough into a 12x14-inch rectangle. With a sharp knife, cut the rectangle in half lengthwise to form two 6x14-inch rectangles. Use a long spatula to help you move one of the dough rectangles onto the parchment-lined baking sheet.
Use a pastry brush to brush a 1-inch border of egg wash around the perimeter of the dough. (Save the remaining egg wash.) Arrange the berries in a 4-inch-wide strip down the length of the dough. Some syrupy juices will likely remain in the dish; spoon 2 to 3 Tbs. over the berries. If some of the liquid seeps onto the egg-washed border, don’t worry about it.
Lightly dust the remaining piece of puff pastry with flour and then gently fold it in half lengthwise; don’t crease the fold. Using a sharp knife, cut 1-1/2-inch-long slashes at 1-inch intervals along the folded side of the dough; leave at least a 1-inch border on the remaining three sides. Do not unfold the dough. Using a long spatula, gently lift the folded strip and position it over the fruit-filled dough rectangle, matching up the straight edges.
Gently unfold the top piece of dough and stretch it over the filling, matching the straight edges all the way around the perimeter of the dough. Press the edges gently with your fingertips to seal the dough, and then, with a fork, very gently crimp the edges of the dough all the way around the pastry.
Bake the jalousie:
Chill the assembled jalousie for 15 to 20 min. Meanwhile, position a rack in the lower third of the oven and heat the oven to 400°F. Brush the top of the jalousie with a light coating of the remaining egg wash (you won't need it all) and sprinkle with the demerara, turbinado or granulated sugar.
Bake for 15 minutes and then rotate the baking sheet. Continue baking until the pastry is puffed, deep golden brown on top and light golden brown on the bottom—use a spatula to gently lift the jalousie so you can peek underneath—10 to 15 min. more. Immediately transfer the jalousie from the baking sheet to a wire rack to cool for at least 45 min. (Instead of trying to move the hot jalousie with a spatula, lift the parchment paper to move the jalousie to the rack and then carefully slide the paper out from under the pastry.)
Serve the jalousie slightly warm or at room temperature with crème fraîche, lightly sweetened whipped cream, or vanilla ice cream, if you like.
Make Ahead Tips
You can refrigerate the filling in a covered container for 2 days before using. The jalousie is best the day it's made, but it will keep, wrapped well in aluminum foil, for 3 days. You can reheat it in a 325°F oven for 5 minutes before serving.
nutrition information (per serving):
based on eight servings;
Photo: Scott Phillips