In a medium bowl, combine the finely diced apples, brown sugar, ground cinnamon, ground nutmeg, and lemon juice. Stir until the apples are evenly coated. Set aside.

Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line with parchment paper, leaving an overhang on the long sides for easy lifting.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until well combined.

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

Pour the wet ingredients into the dry ingredients. Stir with a spatula until just combined. Be careful not to overmix; a few lumps are fine.

Pour about half of the bread batter into the prepared loaf pan and spread it evenly.

Sprinkle about two-thirds of the apple filling mixture over the batter in the pan.

Carefully spoon the remaining bread batter over the apples, spreading it gently to cover as much as possible.

Sprinkle the remaining apple filling over the top layer of batter. Use a knife or a skewer to gently swirl the apple mixture into the top batter a few times, creating a marbled effect.

Bake for 50-60 minutes, or until a wooden skewer or toothpick inserted into the center of the bread comes out with moist crumbs, but no wet batter. If the top starts to brown too quickly, you can loosely tent it with aluminum foil.

Remove the loaf from the oven and let it cool in the pan for 10-15 minutes before carefully lifting it out onto a wire rack to cool completely.

While the bread cools, prepare the glaze: In a small bowl, whisk together the powdered sugar, 2 tablespoons of milk or cream, and vanilla extract (if using) until smooth. Add more milk/cream, a teaspoon at a time, if a thinner consistency is desired.

Once the bread is completely cool, drizzle the glaze generously over the top. Allow the glaze to set for a few minutes before slicing and serving.