Preheat the oven to 350℉ (175℃). Grease and line a 9-inch springform pan with parchment paper. Set aside.
Make the batter. In a small bowl, combine the flour, cinnamon, baking powder, baking soda, salt, and nutmeg. Stir to combine, then set aside.
In a large bowl, combine the granulated sugar, light brown sugar, sourdough discard, eggs, vegetable oil, and vanilla extract. Whisk until smooth.
Add the dry ingredients to the wet ingredients, stirring to combine until no dry spots remain. Fold in the diced apples until incorporated. Spread the batter into the prepared baking pan.
Make the streusel. In a small bowl, combine the flour, oats, light brown sugar, and cinnamon. Add the melted butter and stir until combined and no dry spots remain. The streusel will clump together to form big crumbles.
Evenly spread the streusel on the batter in the baking pan. Sprinkle generously with turbinado sugar (optional).
Bake at 350℉ (175℃) for 45-50 minutes, until the top of the cake is golden brown and the center is firm. You can insert a toothpick in the center of the cake to test for doneness, but be aware that the apples in the cake can make the toothpick appear wet even once the cake is fully baked.
Let the cake cool in the pan for 10 minutes. Run a butter knife around the edges of the pan to separate the cake from the sides, then remove the outer ring. Transfer the cake to a cooling rack to cool.
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Notes
1 serving = 1 slice. While it's not needed for leavening purposes, this recipe can also be made with active sourdough starter. To do so, replace the sourdough discard with active starter (200g). You do not need to make any other changes to the recipe.