Preheat the oven to 350 degrees F (175 degrees C). Line 2 baking sheets with parchment paper.

Grind 2 1/4 cups oats in a food processor to yield 2 cups ground oats. Add walnuts, raisins, flax seeds, cinnamon, baking powder, and salt. Stir in remaining 1 cup oats. Stir thoroughly with a spoon, but do not grind further.

Make a well in the center of oat mixture. Add bananas, eggs, applesauce, honey, and vanilla extract. Mash bananas and stir until dough is well combined.

Drop tablespoonfuls of dough onto the prepared baking sheets. Gently flatten each cookie slightly with the back of a spoon, as they will not spread much during baking.

Bake in the preheated oven until edges are set and the bottoms are lightly golden, about 11 to 15 minutes.