Combine 1/4 cup granulated sugar and cornstarch, stirring well with a whisk. Add mango, juice, and 2 teaspoons melted butter, and toss gently to combine. Place mango mixture in an 8-inch square baking dish coated with cooking spray.
To prepare topping, lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, 3 tablespoons of granulated sugar, brown sugar, and ginger, stirring well. Cut in 3 tablespoons butter with a pastry blender or 2 knives until mixture resembles coarse meal. Stir in nuts. Sprinkle flour mixture evenly over mango mixture. Bake at 400° for 40 minutes or until browned.