Preheat your oven to 375°F (190°C). Line a 9x9 inch baking pan with parchment paper, leaving an overhang on two sides to easily lift the bars out later. Lightly grease the parchment paper.

In a large mixing bowl, combine the all-purpose flour, rolled oats, light brown sugar, granulated sugar, ground cinnamon, and salt. Whisk until well combined.

Add the cold, cubed butter to the dry ingredients. Use a pastry blender, two forks, or your fingertips to cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining. Do not overmix.

Press about two-thirds of the crumble mixture evenly into the bottom of the prepared baking pan. Use the back of a spoon or your hands to firmly press it down to form a compact crust.

In a medium saucepan, combine the diced mango, granulated sugar, lemon juice, cornstarch, and ground ginger (if using). Stir gently to combine.

Cook over medium heat, stirring frequently, for 5-7 minutes, or until the mixture thickens and the mango pieces soften slightly. The filling should be bubbly and jam-like. Remove from heat.

Pour the warm mango filling evenly over the pressed oat crust in the baking pan. Spread it to the edges.

Sprinkle the remaining one-third of the crumble mixture evenly over the mango filling.

Bake for 35-40 minutes, or until the topping is golden brown and the filling is bubbly. If the topping browns too quickly, you can loosely tent the pan with foil.

Remove the pan from the oven and let it cool completely on a wire rack before attempting to cut into bars. This is crucial for the bars to set properly, about 2-3 hours or chill in the refrigerator for 1 hour.

Once cooled and set, use the parchment paper overhangs to lift the bars out of the pan. Place on a cutting board and slice into 12 squares or rectangles.