In a large mixing bowl, combine the flour, rolled oats, sugar, and salt until evenly mixed.

Pour in the melted butter and stir until the mixture resembles moist crumbs.

Reserve about 1 cup of the crumb mixture for the topping.

Firmly press the remaining crumb mixture into the prepared baking pan to create an even crust.

Bake the crust for about 15 minutes until lightly golden around the edges.

While the crust bakes, combine the diced mango, raspberries, sugar, cornstarch, lemon juice, and vanilla extract in a bowl.

Gently toss the fruit mixture until the cornstarch is evenly distributed and the fruit begins releasing juices.

Remove the crust from the oven and allow it to cool for a few minutes.

Spread the fruit filling evenly over the warm crust.

Sprinkle the reserved crumble mixture across the top of the fruit layer.

Return the pan to the oven and bake for 35–40 minutes until the topping is golden brown and the fruit filling is bubbling.

Place the pan on a cooling rack and allow the bars to cool completely.

Refrigerate for at least 1 hour to help the filling set before slicing.

Cut into squares and serve.