Preheat the oven to 350°F (175°C).

In a baking dish, combine diced apples and halved strawberries. Sprinkle in sugar, flour, and kosher salt, and stir gently.

In a bowl, mix oats, flour, brown sugar, white sugar, and optional cinnamon. Cut in the cold butter until it resembles wet sand.

Sprinkle the oat topping evenly over the fruit mixture.

Bake for 45-50 minutes until the topping is golden brown and juices are bubbling.

Let cool for 20-30 minutes before serving, optionally with ice cream or whipped cream.