Position oven racks in the upper and lower thirds of the oven and preheat to 400 degrees F. Line two baking sheets with parchment paper.
While the puff pastry is still mostly frozen but somewhat pliable, unfold it onto a work surface and cut it into 3-by-3/4-inch rectangles. If your puff pastry has seams, cut along these seams and then cut into rectangles. Transfer the rectangles to the prepared baking sheets, leaving about 1/4 inch of space between cookies.
Whisk the sugar, egg white and lemon juice together in a medium bowl until the sugar is fully incorporated and the mixture is thick but spreadable. Using a pastry brush or an offset spatula, evenly spread a thick layer of the sugar mixture onto each cookie, using all the sugar mixture.
Bake the cookies, rotating the sheets back to front and top to bottom halfway through, until the pastry is puffed and golden brown and the sugar topping is golden and shiny, 10 to 14 minutes. Cool on the pans. Serve warm or at room temperature.
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