1. Preheat oven to 350°F (175°C) and grease or line an 8×8-inch baking pan with parchment paper.

2. In a large bowl, whisk melted butter, brown sugar, and granulated sugar until smooth.

3. Beat in eggs one at a time, then stir in sourdough starter discard and vanilla extract.

4. In another bowl, whisk together all-purpose flour, rolled oats, baking powder, baking soda, salt, cinnamon, and nutmeg.

5. Fold the dry ingredients into the wet mixture until just combined, avoiding overmixing.

6. Toss chopped apples and any optional add-ins with a tablespoon of flour, then fold into the batter.

7. Spread the batter evenly in the prepared pan.

8. In a small bowl, mix topping oats, brown sugar, and cinnamon and sprinkle over the batter.

9. Bake for 35 minutes or until a toothpick inserted in the center comes out clean and the top is golden brown.

10. Let the cake cool in the pan for 15 minutes before slicing and serving.