Preheat the toaster oven to 375 degrees F (190 degrees C) according to manufacturer’s instructions. Lightly coat a baking tray with cooking spray.
Pile apple slices onto the baking tray. Mix flour, brown sugar, cinnamon, and salt together; sprinkle evenly over apples. Drizzle water over the topping. Arrange butter pieces as uniformly as possible over the topping.
Bake in the preheated toaster oven until apples are tender, about 30 minutes. Drizzle caramel sauce on top and scoop into a bowl.