Preheat the oven to 180°C / 350°F and line a sheet pan with parchment/baking paper.
Combine the blueberries with sugar, lemon juice, lemon zest, and cornstarch. Mix well then set aside.
On a lightly floured surface, roll the puff pastry out just a little then trim it into a rough circle, approximately 9 inches in diameter. Transfer to the sheet pan lined with baking paper.
Pile the blueberry filling into the center of the pastry then bring the edges in, overlapping as you fold the pastry up around the filling all the way around.
Brush with milk or cream and sprinkle with a little extra sugar.
Bake the galette in the preheated oven for 30-45 minutes or until the pastry is golden brown and crisp and the blueberries are soft, and the filling has thickened slightly.
Remove from the oven and allow to cool for 10 minutes before slicing and serving with ice cream.
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