In large bowl, cream together butter and 1/2 cup sugar. Blend in 3 cups flour and press 1/2 of mixture into the bottom of a 9x13 inch baking dish.
In a medium bowl, stir together mango, 1/4 cup sugar, 1/4 cup flour, cinnamon and nutmeg until well combined. Pour mango mixture over shortbread in pan. Sprinkle remaining shortbread mixture over mango mixture. Press down lightly.
Bake 1 hour, until topping is golden brown. Let cool 15 minutes in dish before cutting into bars.