To make Ginger-Yogurt Topping: Line sieve or colander with cheesecloth, and set over bowl. Spoon yogurt into sieve, cover and let drain in refrigerator at least 30 minutes. Discard liquid. Put drained yogurt and minced ginger in bowl, and stir well. Set aside.

To make Spiced Mango Upside-Down Cake: Preheat oven to 375 degrees F. Put oven rack on lowest position. Spray 10-inch cast-iron skillet or 9-inch cake pan with nonstick cooking spray.

Sprinkle pan with brown sugar. Using back of spoon, press sugar evenly over bottom of pan. Arrange drained mango slices, round side down, on sugar.

Put whole wheat flour, all-purpose flour, baking powder, baking soda, salt, nutmeg and ginger in mixing bowl, and whisk until blended. Put eggs and granulated sugar in separate bowl, and whisk until smooth. Add oil and yogurt, and whisk until smooth. Beat in vanilla. With rubber spatula, fold flour mixture in fourths into yogurt mixture. Pour batter over mango slices in pan; spread evenly.

Bake 30 to 35 minutes, or until top is golden and cake starts to pull away from sides. Immediately loosen edges with knife. Place serving platter over skillet, then invert skillet and platter; remove skillet. Replace any mango slices still in skillet. Cool at least 20 minutes before serving. Serve warm or at room temperature with Ginger-Yogurt Topping. Garnish with crystallized ginger, if desired.