Preheat your oven to 350°F (175°C). Line a large baking sheet with parchment paper or a silicone baking mat to prevent sticking and ensure even browning.
In a medium-sized mixing bowl, whisk together the oat flour, vanilla protein powder, baking powder, and salt. Ensure there are no large clumps of protein powder to maintain a smooth dough consistency.
In a separate large bowl, combine the egg, Greek yogurt, honey, vanilla extract, lemon zest, and lemon juice. Whisk until the mixture is completely smooth and the zest is evenly distributed.
Gradually add the dry ingredients into the wet ingredients. Use a spatula to stir until just combined. Be careful not to overmix, as this can make the cookies tough. The dough will be slightly thicker than muffin batter but stickier than traditional cookie dough.
Gently fold in the fresh blueberries using a spatula. Work carefully to avoid popping the berries, which would turn the dough purple. If using frozen berries, do not thaw them before adding.
Use a cookie scoop or a tablespoon to drop mounds of dough onto the prepared baking sheet. Flatten them slightly with the back of a spoon. Bake for 10 to 12 minutes, or until the edges are lightly golden. Let them cool on the pan for 5 minutes before transferring to a wire rack.