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 until well combined. Scatter the oat topping evenly over the fruit.
Place the dish on a baking sheet coated with foil (for easier clean-up), and bake until the fruit juices are bubbling and the 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.