Preheat the oven to 425ºF. Line a baking sheet with parchment paper or a Silpat.
Pit the peaches then cut them into 1/4-inch-thick slices. In a small bowl, toss together the peaches with the lemon juice.
Unfold the puff pastry onto the baking sheet. Lightly score the dough by cutting a border 1 inch from the edges (do not cut through the pastry). Prick the pastry all over with a fork. Arrange the peach slices so that they are slightly overlapping each other in three rows inside the border.
In a small bowl, whisk together the egg with 2 teaspoons water. Brush the eggwash around the edges of the tart. Bake the tart for 15 to 17 minutes until it is golden and puffed. Remove the tart and let it cool slightly then dust it with confectioners' sugar and serve.
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