📋 Instructions

1. 1Simmer the filling — Combine the blueberries, granulated sugar, cornstarch, lemon juice, and zest in a small saucepan over medium heat. Cook 6 to 8 minutes, stirring and mashing lightly, until the mixture is thick, glossy, and coats the back of a spoon; you should hear a slow, jammy bubble rather than a watery simmer. Scrape onto a plate and cool completely, about 15 minutes.
2. 2Cut the pastry — Unfold the thawed puff pastry sheets on a lightly floured surface and cut each into 3 rectangles about 3 by 4 1/2 inches, giving you 6 tops and 6 bottoms. Keep the pieces you're not working with in the fridge.
3. 3Fill and seal — Spoon about 1 1/2 tablespoons of cooled filling onto the center of each bottom rectangle, leaving a 1/2-inch border. Brush the border with egg wash, lay a top rectangle over it, and press gently to push out any air. Crimp the edges with a fork, then poke three small slits in the top.
4. 4Chill and preheat — Slide the tarts onto a parchment-lined baking sheet and refrigerate 15 minutes while you preheat the oven to 400°F (200°C). Cold pastry hits a hot oven and puffs dramatically, which is exactly what you want.
5. 5Bake until deeply golden — Brush the tops with egg wash and sprinkle generously with sugar. Bake 22 to 26 minutes, until the pastry is deep golden brown, the edges look crisp and layered, and you can see the filling bubbling at the vents. Rotate the pan halfway through if your oven runs uneven.
6. 6Cool before glazing — Let the tarts rest on the pan 10 minutes, then move them to a rack to cool to just warm. Whisk the powdered sugar with the lemon juice until smooth and drizzle over the tops with a spoon, letting it pool and drip down the sides.
7. 7Set and serve — Let the glaze set 10 to 15 minutes, until it's no longer tacky to the touch. Serve slightly warm, when the crust still crackles under your teeth and the filling is just barely gooey.