1. Preheat oven to 400°F. Line a rimmed baking sheet with parchment. In a bowl, combine 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 edges to seal. Repeat with remaining pastry. Transfer pies to baking sheet. Brush with remaining 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.