Coat a 10-inch springform pan with nonstick cooking spray; set aside.
Prepare the crumb topping: In a medium bowl, stir to combine the flour, sugar, cinnamon, salt, and baking powder.
Add the oil and stir thoroughly to combine.
Stir in the walnuts. Set aside.
Prepare the cake batter: In a large bowl, whisk to combine the oil, honey, eggs, sugar, and vanilla.
Add the flour, baking powder, salt, and cinnamon. Whisk until smooth.
Fold in the optional zest, if using.
Pour ½ of the batter into the prepared pan and spread it evenly.
Top with half of the apple slices, fanned out evenly over the surface.
Top with the remaining batter, spreading gently to coat the apples almost to the edge.
Top with the remaining apples fanned out evenly.
Sprinkle the top evenly with crumb topping.
Bake for 60-70 minutes until the cake is golden brown and a toothpick inserted into the center comes out clean. Let the cake cool in the pan on a wire rack for 1 hour.
Remove the cake from the pan and transfer it to a serving platter.
Drizzle the cake with honey and dust lightly with confectioners' sugar, if desired, and serve.