Preheat the oven to 375 degrees. Coat a large baking dish with cooking spray.
Prepare the fruit by tossing the nectarines and blueberries with sugar, cornstarch, zest, and salt.
Pour the fruit mixture into the prepared baking dish.
Make the topping by combining the oats, flour, butter, and sugar until well combined. Scatter oat topping evenly over the fruit.
Place the dish on a baking sheet coated with foil (for easier clean-up), and bake until fruit juices are bubbling and the topping is golden brown, 45-50 minutes.
Transfer to a wire rack and let cool for an hour. Serve as is or topped with vanilla ice cream or whipped cream. Enjoy.