1. Preheat your oven to 350 F (175 C). Grease and flour an 8×8 inch square baking pan or a 9-inch round cake pan. You can also line it with parchment paper for easy removal.

2. In a large bowl, whisk together the all-purpose flour, granulated sugar, brown sugar, baking soda, ground cinnamon, ground nutmeg, and salt. Make sure there are no lumps, especially in the brown sugar.

3. In a separate medium bowl, whisk together the melted butter, sourdough discard, egg, and vanilla extract until well combined and smooth.

4. Pour the wet ingredient mixture into the dry ingredient bowl. Stir with a spatula or wooden spoon until just combined. Be careful not to overmix; a few streaks of flour are okay. Overmixing can lead to a tough cake.

5. Gently fold in the finely diced apples until they are evenly distributed throughout the batter.

6. Pour the batter into your prepared baking pan and spread it evenly. If desired, sprinkle the top with turbinado sugar or cinnamon sugar for a lovely crust.

7. Bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The top should be golden brown and spring back lightly when touched.

8. Let the cake cool in the pan for at least 10-15 minutes before slicing and serving. This allows it to set properly and makes for easier removal.