Heat oven to 375° F with the rack in the lowest position.
Place the piecrust on a parchment-lined baking sheet. Leaving a 2-inch border, top with the nectarines or peaches. Sprinkle with 2 tablespoons 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 2 teaspoons water and sprinkle with the remaining tablespoon of sugar.
Bake until the juices are bubbling and the crust is golden (tent with foil if the crust browns too quickly), 35 to 40 minutes.