Place flour, salt and cubed butter into a food processor. Pulse until mixture resembles pea-sized crumbles. Add cold water 1 tbs. at a time until mixture forms a smooth ball. Set aside.
In a medium-sized bowl, gently combine nectarine slices, raspberries, blackberries, sugar, flour and lemon juice.
On a large baking sheet or pizza stone, sprinkle a pinch of flour and roll out pastry dough into a 16-inch circle.
Spread filling evenly onto pastry, leaving a 1 1/2-inch dry outside edge. Carefully pull up edge of pastry and fold over outside edge of the filling all around. Remember, it is meant to look rustic, so don't worry about perfection. Sprinkle outside edges of pastry with sugar.
Bake at 375 F for 40-45 minutes or until pastry is lightly browned.
Allow to cool for 20-30 minutes. Slice into eighths and serve warm.