1. Prepare Your Muffin Tin: Start by preheating your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or grease it thoroughly with butter or non-stick spray. This prevents sticking and makes for easy removal.

2. Combine Wet Ingredients: In a large mixing bowl, whisk together the active sourdough starter, milk, melted butter, egg, brown sugar, and vanilla extract until the mixture is smooth and well combined. Ensure the butter isn’t too hot, or it might cook the egg.

3. Mix Dry Ingredients Separately: In a separate medium bowl, combine the all-purpose flour, old-fashioned rolled oats, baking powder, baking soda, and salt. Stir these dry ingredients together thoroughly with a whisk to ensure even distribution.

4. Combine Wet and Dry Mixtures: Gradually add the dry ingredient mixture to the wet ingredient mixture. Stir gently with a spoon or spatula until just combined. Be careful not to overmix, as this can lead to tough muffins. A few lumps are perfectly fine.

5. Fold in Blueberries: Gently fold in the fresh blueberries. If using frozen blueberries, add them directly from the freezer without thawing. Distribute them evenly throughout the batter.

6. Fill Muffin Cups: Divide the batter evenly among the 12 prepared muffin cups, filling each about two-thirds full. For a nice domed top, you can fill them a bit more generously.

7. 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.

8. Cool and Serve: Let the muffins cool in the muffin tin for a few minutes before transferring them to a wire rack to cool completely. This helps prevent them from becoming soggy.