1. Preheat your oven to 375 F (190 C). Lightly grease an 8×8-inch baking dish or line it with parchment paper, leaving an overhang on the sides.

2. In a large mixing bowl, whisk together the rolled oats, all-purpose flour, light brown sugar, baking powder, and salt until well combined.

3. In a separate medium bowl, combine the active sourdough starter, melted unsalted butter, large egg, and vanilla extract. Whisk until thoroughly blended and smooth.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Stir gently with a spoon or spatula until just combined. Do not overmix.

5. Gently fold in the fresh or frozen blueberries. If using frozen, add them directly from the freezer.

6. Spoon the batter evenly into your prepared 8×8-inch baking dish. Use the back of a spoon or a spatula to gently press the mixture down into an even layer.

7. Bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out mostly clean. The bars should feel firm to the touch.

8. Remove the dish from the oven and let the bars cool completely in the pan on a wire rack before attempting to slice them.