Mix sliced mangoes with 1 teaspoon of the cinnamon, the nutmeg, and lemon juice. Place in a buttered 10 x 6 x 2 inch glass baking dish. Set aside. Combine butter and brown sugar until well blended.
Cut in flour and the remaining 1/2 teaspoon of cinnamon. Mix until crumbly. Sprinkle the topping evenly over mango slices. Cook in microwave oven 9 to 11 minutes on HIGH power, rotating a half turn after 5 minutes. Serve warm or cold, with whipped topping or ice cream, if desired.