Preheat your oven to 350°F (175°C). Line an 8-inch square pan with foil, leaving overhang for easy removal, and lightly grease with non-stick spray or butter.

In a large mixing bowl, melt ½ cup of unsalted butter. Add in 1 cup all-purpose flour, 1 cup oats, ½ cup granulated sugar, ½ cup light brown sugar, and a pinch of salt. Stir until well combined.

Scoop out 1 cup of the crumble mixture and set aside for topping.

Press the remaining mixture firmly into the bottom of the prepared pan.

In another bowl, combine 2 cups coarsely chopped mango, ¼ cup sugar, 1 tablespoon lemon juice, and 1 tablespoon corn starch. Toss until well-coated.

Spread the mango mixture evenly over the crust in the pan. Sprinkle the reserved crumble on top.

Bake for 50 minutes until golden brown and bubbling.

Let cool for at least 1 hour before slicing into squares.