Preheat the oven to 350 degrees. Melt the butter in an 8 or 9-inch cast iron skillet or ovenproof pan. Set aside.
In a small bowl, whisk together the flour, baking soda, cinnamon and ginger. Set aside. In a large bowl, whisk together the egg, grated fresh ginger, ½ cup maple syrup, sugar, and yogurt.
Stir in 2 tablespoons melted butter. Add the dry ingredients, whisking to blend.
To prepare the apples, set a peeled apple on a cutting board, stem side up. Cut a large piece from one side, leaving the core. Rotate the apple and repeat twice for a total of 3 large slices from each apple.
To the remaining 2 tablespoons butter in the skillet, add ¼ cup maple syrup. Add the apples, stir to coat. Cook the apples in the syrup, turning, until they soften, about 10 minutes.
Space apples evenly in the pan, flat side down. Spoon batter over the top and smooth. Bake until a toothpick comes out clean, about 45 minutes. Let cool on a rack in the pan for 10 minutes before inverting onto a plate.
Serve with a dollop of yogurt or whipped cream. Makes 8 servings.