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Heat oven to 400 degrees, and position a rack in the lower third of the oven. Line a rimmed baking sheet with parchment paper.
Combine sugar, lemon zest and vanilla in a large bowl. Use your fingertips to rub the zest and seeds (or extract) into the sugar. Add the flour and salt and stir to combine. Add the nectarines, blueberries and lemon juice, but don’t stir quite yet.
On a lightly floured surface, roll dough into a 12-inch round just under 1/4-inch thick. Transfer the dough to the prepared baking sheet.
Stir the filling to combine, then pile all the fruit in the center of the dough, leaving a 2-inch border. Press the fruit gently to make sure there are no large open pockets. Fold the edges of the dough up and over the fruit and press gently to seal the folds.
Put the whole baking sheet into the freezer, and chill the formed galette until the dough is very firm, about 15 minutes. Brush dough with the beaten egg, and sprinkle with sugar and salt. Bake until deep golden brown and the juices are bubbling, about 45 minutes. Cool slightly before serving.