To prepare the filling: in a medium saucepan, stir together sugar, cornstarch, ginger, and nutmeg. Stir in the water. Cook and stir until thickened. Gently stir in orange and grapefruit sections.
To prepare the topping: in a medium bowl, stir together flour, sugar, the baking powder, ginger, and salt. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. In a small bowl, stir together yogurt, egg, and milk. Add yogurt mixture to flour mixture, stirring just until moistened.
Spoon filling into a 2-quart square baking dish. Using two spoons, drop the topping into nine mounds onto hot filling.
Bake for 20 to 25 minutes or until a wooden toothpick inserted into topping comes out clean. Cool slightly. If desired, sprinkle with toasted coconut and/or top with dessert topping. Serve warm.
Tip: Sugar Substitutes: Choose from Splenda® Granular, Sweet'N Low® bulk or packets, Truvia®, or Nevella®. Follow package directions to use product amount equivalent to 3 tablespoons and 1 tablespoon sugar. Nutrition Facts Per Serving with Substitute: same as below, except 153 cal., 29 g carb.
To toast coconut, spread in a shallow baking pan lined with parchment paper. Bake in a 350°F oven for 5 to 10 minutes or until golden, shaking pan once or twice, watching closely to avoid burning.