Preheat the oven to 180°C fan forced. Grease four, 1½ cup capacity ovenproof ramekins or one, 5cm deep, 20x30cm (base) baking dish.
Cut the mango cheeks from the stones. Using a large spoon, remove the mango fruit from the cheeks and thickly slice the fruit. Place into a bowl, add the raspberries. Sprinkle over the raw sugar, stir gently to coat (this prevents the mango from breaking down and stewing). Spoon the fruit and any juices between the ramekins or baking dish.
Dollop the custard over the fruit.
Sift the flour, baking powder and cinnamon into a bowl. Add the butter. Using fingertips, rub the butter into the flour mixture until the mixture resembles coarse breadcrumbs. Stir in the sugar and oats and use your fingertip to mix until the crumble starts to clump together.
Spoon the crumble mixture evenly over the fruit and custard. Place the ramekins on a baking tray. Bake in for 35-40 minutes or until the crumble is golden. Dust with icing sugar, serve at room temperature with ice cream.