How to Make It
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.
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.
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.