1. Prepare your baking dish and preheat the oven: Start by preheating your oven to 350 F (175 C). Lightly grease an 8×8 inch baking dish with cooking spray or line it with parchment paper, leaving an overhang on the sides to easily lift the bars out later. This prevents sticking and makes for clean removal.

2. Combine the dry ingredients: In a large mixing bowl, whisk together the rolled oats, vanilla protein powder, all-purpose flour, baking powder, and salt. Make sure there are no lumps, and everything is evenly distributed. This ensures a consistent texture throughout your bars.

3. Mix the wet ingredients: In a separate medium bowl, whisk together the unsweetened applesauce, maple syrup, milk, large egg, vanilla extract, and lemon zest until well combined. The mixture should be smooth and uniform.

4. Combine wet and dry mixtures: Pour the wet ingredients into the bowl with the dry ingredients. Stir everything together with a spatula until just combined. Be careful not to overmix; a few lumps are fine. Overmixing can lead to tougher bars.

5. Gently fold in the blueberries: Add the fresh or frozen blueberries to the batter. Gently fold them in until they are evenly distributed throughout the mixture. Avoid mashing the blueberries, especially if they are fresh, to keep them intact.

6. Spread into the baking dish: Pour the batter into your prepared 8×8 inch baking dish. Use the back of your spatula or a spoon to spread the mixture evenly into the pan, ensuring it reaches all corners. Lightly press down to create a compact layer.

7. Bake until golden: Place the baking dish in the preheated oven and bake for 20-25 minutes, or until the edges are lightly golden brown and a toothpick inserted into the center comes out clean. The bars should feel firm to the touch.

8. Cool completely before slicing: Once baked, remove the dish from the oven and let the bars cool completely in the pan on a wire rack. This is crucial for them to set properly and prevent crumbling when you slice them. Once cool, use the parchment paper overhang to lift them out and cut them into 9 or 12 bars.