How to Make It
In a large bowl, whisk the flour with the oats, pecans (if using), brown sugar, cinnamon, ginger and salt. With your hands, rub the butter into the mixture until incorporated, pressing it into clumps. Add the pears and toss well. Refrigerate the topping until chilled, about 20 minutes.
Preheat the oven to 350° and grease a 9-by-13-inch metal baking pan. In a bowl, whisk the flour, baking powder, baking soda and salt. In a stand mixer fitted with the paddle, beat the 2 sticks of butter with the sugar at medium speed until fluffy, 2 minutes. Beat in the eggs one at a time, then beat in the sour cream and vanilla. Scrape down the side of the bowl, then beat the dry ingredients into the batter in three additions until just incorporated.
Scrape the batter into the prepared pan, spreading it in an even layer. Cover with the streusel-pear topping. Bake for 1 hour and 15 minutes, until the crumb topping is browned and a toothpick inserted in the center of the cake comes out clean. Transfer the pan to a rack and let the cake cool completely, about 1 hour. Dust the cake with confectioners' sugar, cut into squares and serve.