Start by preheating your oven to 180°C (350°F). Grease a 8.5 x 10 baking dish with butter or cooking spray.
Prepare the crumble topping. In a large mixing bowl, combine the all-purpose flour, oats, light brown sugar, and ground cinnamon. Cut in the melted unsalted butter using a fork until the mixture resembles coarse crumbs. Set aside.
Peel, core, and chop the apple into chunks. If you prefer a stronger strawberry flavor, you can grate the apple instead. Wash and cut the strawberries, leaving them whole if they're small or halving/quartering them if they're larger.
In a large bowl, add the chopped apple, strawberries, granulated sugar, vanilla extract, all-purpose flour, and water to create a syrupy consistency. Mix well until all the ingredients are combined.
Transfer the fruit mixture to the greased baking dish and spread it out evenly.
Sprinkle the prepared crumble topping over the fruit, making sure to cover the entire surface.
Place the baking dish in the preheated oven and bake for about 40-45 minutes or until the crumble topping is golden brown and the fruit is bubbly around the edges.
Once baked, remove it from the oven and let it cool slightly before serving.