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

2. In a large mixing bowl, whisk together the all-purpose flour, rolled oats, granulated sugar, brown sugar, baking powder, salt, and protein powder. Make sure there are no lumps and everything is evenly distributed.

3. Pour the melted butter, egg, and vanilla extract into the dry ingredient mixture. Mix with a fork or your hands until a crumbly dough forms. It should be moist enough to hold together when pressed.

4. Press about two-thirds of the dough mixture evenly into the bottom of the prepared baking pan. Use the back of a spoon or your fingers to create a firm, compact base.

5. In a separate medium bowl, gently combine the fresh blueberries, lemon zest, fresh lemon juice, and cornstarch. Toss until the blueberries are evenly coated. The cornstarch will help thicken the filling as it bakes.

6. Spread the blueberry filling evenly over the pressed dough base in the pan. Crumble the remaining one-third of the dough mixture over the blueberry layer. Do not press this top layer down; keep it loose and crumbly for a nice texture.

7. Place the baking pan in the preheated oven and bake for 30-35 minutes, or until the top is lightly golden brown and the blueberry filling is bubbly. A good visual cue is when the edges start to pull away slightly from the pan.

8. Remove the pan from the oven and let the bars cool completely on a wire rack before attempting to cut them. This is crucial for clean slices and to allow the bars to set properly. Once cooled, use the parchment paper overhang to lift the entire slab out of the pan and cut into squares or rectangles.