Preheat the oven to 350 degrees. Coat a 10-inch cast iron skillet with cooking spray.
Prepare the fruit by tossing the strawberries, nectarines, and raspberries with sugar, cornstarch, lemon juice, and salt.
Pour the fruit mixture into the prepared cast iron skillet or baking dish.
Prepare the oat topping by combining the oats, flour, butter, sugar, cinnamon, and salt together until well combined. Scatter oat topping evenly over the fruit.
Bake for 40-45 minutes, or until the fruit is bubbling and the oat topping is golden brown.
Remove from the oven and allow to cool for 15-30 minutes before serving.
Serve plain, with whipped cream, or with ice cream. Enjoy!