I created this recipe as another way to use extra sourdough starter. But you do not need to be a sourdough baker to make this moist whole-grain coffee cake. See the notes for a simple substitution.
To make the streusel, combine the butter, brown sugar, flour, allspice, walnuts and salt in a small bowl until well combined and set aside.
Preheat oven to 375°F. Grease an 8-inch square baking dish or round cake pan. Whisk the flour, brown sugar, ginger, cinnamon, salt, baking powder, baking soda, in a mixing bowl to blend well.
Measure the oil in a glass measuring cup and beat in the egg. Add to the flour mixture and stir, the add the starter and blend well to make a thick batter. Fold in the pears and pour into the prepared baking dish.
Crumble the reserved streusel topping all over the batter. Bake until the cake is springy to the touch and a wooden skewer inserted in the center comes out clean, about 35 minutes. Cool on a baking rack before cutting into serving-size pieces.
In place of the sourdough starter, combine 3/4 cup buttermilk (or use yogurt thinned with milk if you don't have buttermilk on hand) with 1/2 cup all-purpose flour. Blend and add to the batter to replace the sourdough starter.