Preheat your oven to 350 degrees F. Grease a 12-hole muffin pan with coconut oil or your preferred cooking oil to prevent sticking.
In a mixing bowl, whisk together the sourdough starter, egg, coconut oil, and vanilla extract. Continue whisking until everything is very well-combined with no visible oil separation.
In a separate bowl, combine the gluten-free all-purpose flour, coconut sugar, baking soda, and sea salt. Make sure to mix these dry ingredients thoroughly.
Slowly add the dry mixture to the wet mixture in the mixing bowl. Stir gently until everything is well combined. The batter will be quite thick.
Now, fold in the fresh blueberries gently, being careful to maintain the integrity of the berries.
Fill each muffin hole about two-thirds to three-quarters full with the batter.
In a small bowl, combine the topping ingredients: coconut oil, coconut sugar, and gluten-free all-purpose flour. Mix well until crumbly and sprinkle generously over the filled muffin batter.
Transfer the muffin pan to the center rack of your preheated oven. Bake for 20 to 25 minutes.
Once baked, allow the muffins to cool in the pan for at least 20 minutes. Gently run a butter knife around the edges to help release them from the pan.
Enjoy your Gluten Free Blueberry Sourdough Muffins warm, or store them in an airtight container for later.