Preheat your oven to 350°F (175°C) and line an 8-inch square pan with foil, greasing it lightly.

Melt the unsalted butter in a microwave-safe bowl for about 30 seconds until fully liquefied.

Add the all-purpose flour, old-fashioned rolled oats, granulated sugar, light brown sugar, and salt to the melted butter and mix until crumbly, reserving a heaping cup for topping.

Press the remaining mixture firmly into the bottom of the prepared pan to form the crust.

In a separate bowl, combine the chopped mango, granulated sugar, lemon juice, and corn starch; toss gently to coat.

Spread the mango mixture evenly over the crust in the pan.

Sprinkle the reserved crumble topping evenly over the mango layer.

Bake for approximately 50 minutes until edges are set and filling bubbles around the edges.

Let cool at room temperature for at least 1 hour before slicing.