Preheat oven to 350F. Line a baking sheet with parchment paper or spray with cooking spray.
In a small bowl, stir together flax and water. Set aside.
Place banana in a medium mixing bowl and mash with a fork. Add yogurt and vanilla extract, stirring to combine. Stir in flax egg.
Add almond meal, oat flour, oats, baking soda, cinnamon, and salt. Mix until just combined. Fold in walnuts and blueberries.
Using wet hands, roll mixture into 12 large balls of “dough”. They will be wet and sticky–that’s ok! Wet hands again to pat down cookies until they look round and smooth. Lightly flatten with hands. They won’t spread much.
Bake for 20 minutes. Store cooled cookies in an air-tight container in the fridge.