1. Preheat the oven to 350°F (175°C) and grease and line a 9×9-inch baking pan with parchment paper.

2. In a large bowl whisk together melted butter, brown sugar, eggs, and vanilla extract until smooth.

3. Add milk and sourdough starter to the wet mixture and stir until combined.

4. In a separate bowl whisk flour, baking powder, baking soda, salt, cinnamon, and nutmeg.

5. Gradually fold the dry ingredients into the wet ingredients until no streaks remain.

6. Fold in diced apples until evenly distributed in the batter.

7. Allow the batter to rest for 15 minutes if time allows.

8. In a small bowl combine flour, brown sugar, cinnamon, salt, butter, and nuts if using, and rub with fingers until crumbly.

9. Spread the batter into the prepared pan and sprinkle the crumble topping evenly over the surface.

10. Bake for 40 to 50 minutes until a toothpick inserted in the center comes out clean.

11. Cool the cake in the pan on a wire rack for 20 minutes before slicing.

12. Serve warm with coffee or whipped cream if desired.