How to Make It
Preheat the oven to 325°. On a parchment paper—lined baking sheet, toast the pecans for about 12 minutes, until lightly browned and fragrant. Let cool. In a medium skillet, cook the oats over moderate heat, tossing frequently, until fragrant and lightly toasted, about 4 minutes. Let cool.
In a food processor, pulse the flour with the toasted pecans until the nuts are finely ground. Transfer to a medium bowl and stir in the oats, light brown sugar, granulated sugar, salt, black pepper and cinnamon. Using a fork, stir in the butter until large clumps form. Spread the streusel on the parchment-lined baking sheet in a single layer and bake for about 25 minutes, until golden brown. Let cool.
Increase the oven temperature to 350°. Generously butter and flour a 10-inch Bundt pan. In a large bowl, combine 1/4 cup of the sugar with the cornstarch, nutmeg, cinnamon and allspice. Add the apples and toss until the apples are thoroughly coated.
In a large skillet, melt 3 tablespoons of the butter. Add the spiced apples and cook over moderately high heat, stirring occasionally, until they're almost tender, about 6 minutes. Scrape the apples onto a baking sheet and refrigerate until they are cool, about 15 minutes.
In a medium bowl, whisk the flour with the baking soda and salt. In a measuring cup, mix the buttermilk with the Calvados and vanilla. In the bowl of a stand mixer fitted with the paddle, beat the remaining 2 sticks of butter and 2 cups of sugar with the orange zest at medium-high speed until fluffy, about 2 minutes. Add the eggs one at a time, beating well between additions. At low speed, in 3 alternating additions, beat in the dry ingredients and buttermilk mixture; scrape down the side of the bowl as necessary.
Spoon the cooled apples into the bottom of the Bundt pan, spreading them out. Scrape the cake batter over the apples and smooth the surface with a spatula. Bake the cake in the center of the oven for about 1 hour, until a toothpick inserted in the center of the cake comes out clean. Let the cake cool for 5 minutes.
Set a cooling rack on a baking sheet. Invert the cake onto the rack and scrape any apples stuck to the pan back onto the cake. While the apples are hot, pack the streusel topping over the top of the cake. Let stand until cool, about 1 hour. Slice the cake with a serrated knife and serve.