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

2. In a large mixing bowl, whisk together the gluten-free all-purpose flour blend, gluten-free rolled oats, ground flax seeds, chia seeds, sunflower seeds, baking powder, baking soda, and salt. Make sure there are no lumps and all ingredients are evenly distributed.

3. In a separate medium bowl, whisk together the active gluten-free sourdough starter, milk, melted butter, maple syrup, and egg until well combined and smooth.

4. Pour the wet ingredients into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few lumps are perfectly fine and will help keep the muffins tender.

5. Gently fold in the fresh or frozen blueberries. If using frozen blueberries, do not thaw them first, as this can cause the batter to turn purple. Fold them in quickly to avoid crushing them.

6. Divide the batter evenly among the 12 prepared muffin cups, filling each about two-thirds full. If desired, sprinkle a little turbinado sugar on top of each muffin for a sweet, crunchy crust.

7. Bake for 20-25 minutes, or until the muffins are golden brown and a wooden skewer or toothpick inserted into the center of a muffin comes out clean.

8. Let the muffins cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely. Enjoy them warm or at room temperature.