1.Heat oven to 375°F with the rack in the lowest position
2.Place the piecrust on a parchment-lined baking sheet. Leaving a 2-in border, top with the nectarines or peaches. Sprinkle with 2T of the sugar and the butter. Fold the edges of the dough toward the center, overlapping slightly and partially covering the fruit. Brush the dough with the 2tsp water and sprinkle with the remaining tablespoon of sugar.
3.Bake until the juices are bubbling and the crust is golden (tent with foil if the crust browns too quickly), 35 to 40 minutes.
The galette can be baked up to 8 hours ahead. It’s wonderful served at room temperature with a scoop of vanilla ice cream.