Preheat the oven to 375 degrees. Coat a baking dish with cooking spray.
Toss the blackberries and nectarines with sugar, cornstarch, cinnamon, and salt.
Pour the fruit mixture into the prepared baking dish.
Combine the oats, almond slices, flour, butter, sugar and cinnamon together until well combined. Scatter oat topping evenly over the fruit.
Place the dish on a baking sheet coated with tinfoil (for easier clean-up), and bake until fruit juices are bubbling and topping is golden brown, about 45-55 minutes.
Transfer to a wire rack and let cool for an hour.
Serve as is or topped with whipped cream or vanilla ice cream . Enjoy.