5 tablespoons unsalted butter, cut into 1/2-inch pieces
Preheat the oven to 425°. Generously butter a large baking dish. Add the rhubarb and mango, then add 6 1/2 tablespoons of the sugar and toss gently to combine.
In a food processor, combine the remaining 2 tablespoons of sugar with the flour, cornmeal, ginger, white pepper and salt; pulse a few times to mix. Add the butter and process just until the mixture begins to clump. Transfer the topping to a bowl. Pinch the topping to form large and small crumbs and sprinkle them evenly over the fruit. Bake for 25 to 30 minutes, or until bubbly and golden on top. If desired, set the baking dish under the broiler for a few seconds to brown the top.