1. Preheat oven to 400ºF. Line a rimmed baking sheet with parchment. In a bowl, stir nectarine, berries and 2 Tbsp. sugar. On another piece of parchment, cut puff pastry into four equal squares. In a bowl, whisk egg with 1 tsp. water.
2. Brush edges of a pastry square with egg wash. Spoon 1/4 of fruit mixture onto center of pastry. Fold pastry diagonally to form a triangle. Using a fork, press to seal edges. Repeat with remaining pastry and fruit. Transfer pies to baking sheet. Brush tops with egg wash; sprinkle with 1 tsp. sugar. Cut a small slit in top of each pie.
3. Bake until pies are golden brown, 20 to 25 minutes, rotating sheet from front to back midway through. Allow pies to cool on a wire rack for at least 10 minutes before serving.