1. Preheat your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or lightly grease each cup.

2. In a large mixing bowl, combine 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, baking powder, baking soda, 1/2 teaspoon ground cinnamon, nutmeg, and 1/4 teaspoon salt. Whisk to thoroughly combine.

3. In a separate medium bowl, whisk together the sourdough starter, milk, melted and cooled 1/4 cup unsalted butter, egg, and vanilla extract until well combined and smooth.

4. Pour the wet ingredient mixture into the dry ingredients. Stir with a spatula or wooden spoon just until combined. Do not overmix; a few lumps are fine.

5. Gently fold the finely diced apples into the batter, distributing them evenly without overworking the batter.

6. To prepare the streusel topping, in a small bowl, combine 1/4 cup all-purpose flour, 2 tablespoons granulated sugar, brown sugar, and 1/4 teaspoon ground cinnamon. Add the cold, cubed 2 tablespoons unsalted butter. Using your fingertips or a pastry blender, cut the butter into the dry ingredients until the mixture resembles coarse crumbs.

7. Divide the muffin batter evenly among the prepared muffin cups, filling each about two-thirds full. Sprinkle a generous amount of streusel topping over the top of each muffin.

8. Bake for 20-25 minutes, or until the muffins are golden brown and a wooden skewer or toothpick inserted into the center comes out clean. Let them cool in the muffin tin for a few minutes before transferring them to a wire rack to cool completely.