Preheat the oven to 350°F (175°C) to ensure even baking and juicy fruit.

In a mixing bowl, combine mixed fresh fruit with granulated sugar and lemon juice to draw out juices and add brightness.

Spread the fruit mixture evenly in the prepared 9-inch oven-safe baking dish.

In another bowl, mix all-purpose flour, rolled oats, brown sugar, and cinnamon to create the crumble topping base.

Add softened unsalted butter to the dry topping mixture and use your fingers to combine until crumbly.

Sprinkle the crumble topping evenly over the fruit layer for a golden-brown finish.

Bake in the preheated oven at 350°F (175°C) for 30 minutes, until the topping is golden and the fruit is bubbling.

Remove from the oven and let cool for a few minutes before serving to allow the juices to firm up.