1. Preheat your oven to 350 F (175 C). Lightly grease and flour an 8×8 inch square baking pan, or line it with parchment paper, leaving an overhang on two sides for easy lifting.

2. In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth. Beat in the egg and vanilla extract until well combined and the mixture is light and creamy. Stir in the sourdough discard until fully incorporated. The mixture should be smooth with no lumps of discard.

3. In a separate medium bowl, whisk together the all-purpose flour, rolled oats, baking powder, baking soda, ground cinnamon, ground nutmeg, and salt. Make sure all dry ingredients are evenly distributed to ensure proper leavening.

4. Gradually add the dry ingredient mixture to the wet ingredient mixture, mixing on low speed or by hand 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. The batter will be thick, but that’s perfectly normal.

6. Pour the batter into your prepared 8×8 inch baking pan and spread it evenly. If desired, sprinkle the top with turbinado sugar for a lovely crunchy crust. Bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. The cake should be golden brown and spring back lightly when touched.

7. Let the cake cool in the pan on a wire rack for at least 15-20 minutes before slicing and serving. This allows the cake to set properly and makes for easier removal from the pan.