1. Prepare Your Baking Pan and Oven: Preheat your oven to 350 F (175 C). Grease and lightly flour (with gluten-free flour) a 9×5-inch loaf pan, or line it with parchment paper, leaving an overhang on the long sides to easily lift the loaf out later.

2. Combine Dry Ingredients: In a large mixing bowl, whisk together the gluten-free all-purpose flour blend, granulated sugar, baking powder, baking soda, and salt until well combined. This ensures all the leavening agents are evenly distributed.

3. Mix Wet Ingredients: In a separate medium bowl, whisk together the sourdough discard, milk, melted butter (or oil), egg, and vanilla extract until smooth. Make sure the butter isn’t too hot, or it might cook the egg.

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

5. Fold in Blueberries: If using, gently toss the blueberries with 1 tablespoon of gluten-free flour in a small bowl. Then, carefully fold the blueberries into the batter until they are evenly distributed.

6. Transfer to Pan and Bake: Pour the batter into the prepared loaf pan and spread it evenly. Bake for 50-60 minutes, or until a wooden skewer or toothpick inserted into the center of the loaf comes out clean. The top should be golden brown.

7. Cool Before Slicing: Once baked, remove the loaf pan from the oven and let it cool in the pan for about 10-15 minutes. Then, carefully lift the loaf out using the parchment paper overhang or invert it onto a wire rack to cool completely before slicing. This prevents the loaf from crumbling.