6 medium Bosc pears (about 3 1/3 pounds)
6 tablespoons (3 ounces) unsalted margarine
¾ cup packed light brown sugar
1-1/3 cups (6 ounces) flour
2/3 granulated sugar
3 tablespoons minced crystallized ginger
1 ½ teaspoons ground cinnamon
1 teaspoon baking soda
½ teaspoon ground ginger
½ teaspoon table salt
3 large eggs
½ cup canola oil
1 teaspoon pure vanilla extract
1 teaspoon finely grated orange zest
Position a rack in the center of the oven and heat to 350 degrees.
Peel, quarter, and core 4 of the pears and set aside. Peel and grate the other 2 and set them aside separately.
In a 10 inch cast iron skillet, melt the margarine over low heat. Remove from the heat and sprinkle the brown sugar over the margarine. Arrange the quartered pears on the sugar; if necessary, trim a few pieces to fit and fill center.
Combine the flour, granulated sugar, crystallized ginger, cinnamon, baking soda, ground ginger, and salt in a medium bowl and whisk to blend. In a separate large bowl, whisk the eggs, oil, vanilla, and orange zest until blended. Stir in the grated pears. Add the flour mixture and stir just until blended.
Pour the batter over the pears and smooth to the top. Bake until the cake is deep golden brown and a toothpick inserted into the center comes out clean, about 40 minutes. Cool the cake in the skillet for 20 minutes, then run a knife around the edge of the pan and turn out onto a plate. Serve warm or at room temperature.