Prepare the Muffin Batter: Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease the tin with butter or cooking spray. In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy. Beat in the eggs, one at a time, followed by the vanilla extract. Add the sourdough starter and milk, and mix until everything is well combined.

1. Add the Dry Ingredients: In a separate bowl, whisk together the flour, cinnamon, nutmeg, baking powder, baking soda, and salt. Gradually fold the dry ingredients into the wet ingredients until just combined. Be careful not to overmix.
2. Add Apples and Nuts: Gently fold in the diced apples and optional nuts.
3. Make the Streusel Topping (Optional): In a small bowl, combine the flour, sugar, cinnamon, and cold cubed butter. Use a pastry cutter or your fingers to work the butter into the dry ingredients until the mixture forms small crumbs.
4. Fill the Muffin Tin: Divide the muffin batter evenly among the muffin cups, filling each about 3/4 full. If using the streusel topping, sprinkle it generously over the batter in each cup.
5. Bake: Bake the muffins in the preheated oven for 18-22 minutes, or until the tops are golden brown and a toothpick inserted into the center comes out clean. Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.
6. Serve: Serve the muffins warm, or store them in an airtight container for up to 3 days. These muffins are delicious on their own or with a dollop of butter or jam.