1. 1. Prepare the Overnight Batter: In a large mixing bowl, combine the unfed sourdough starter, milk, melted butter, and egg. Whisk these wet ingredients together until they are well combined and smooth.

2. 2. Add Dry Ingredients: In a separate medium bowl, whisk together the granulated sugar, all-purpose flour, baking powder, baking soda, and salt. Make sure there are no lumps and the leavening agents are evenly distributed.

3. 3. Combine Wet and Dry: Pour the dry ingredients into the wet ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few lumps are perfectly fine. Overmixing can lead to tough muffins.

4. 4. Fold in Blueberries: Gently fold in the blueberries. If using frozen blueberries, add them directly from the freezer. Stir just enough to distribute them throughout the batter.

5. 5. Refrigerate Overnight: Cover the bowl tightly with plastic wrap and refrigerate for 8 to 12 hours. This overnight rest allows the sourdough to work its magic, developing flavor and tenderizing the batter.

6. 6. Preheat and Prepare Muffin Tin: The next morning, preheat your oven to 400 F (200 C). Line a 12-cup muffin tin with paper liners or grease it well with butter or non-stick spray.

7. 7. Fill Muffin Cups: Remove the batter from the refrigerator. Give it a quick, gentle stir. Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full. If desired, sprinkle the tops with turbinado sugar for a lovely crunch.

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