Preheat the oven to 350°F and line a baking sheet with parchment paper.

In a medium bowl, mash the ripe banana until mostly smooth.

Gently fold the mixed berries into the mashed banana.

In a large bowl, whisk the egg until blended.

Add the banana-berry mixture to the egg and stir until combined.

Add the oat flour gradually, stirring until a soft, workable dough forms.

If using, sprinkle in the ground flaxseed and mix until evenly incorporated.

If the dough is too sticky, add a little more flour; if too dry, add a teaspoon of water and mix again.

Lightly flour a clean work surface and roll the dough to about 1/4-inch thickness.

Use a small cookie cutter or knife to cut the dough into bite-size shapes.

Arrange the cut treats on the prepared baking sheet, spacing them slightly apart.

Bake for 20–25 minutes, or until the treats are firm and lightly golden on the bottom.

Remove the pan from the oven and let the treats cool completely on a wire rack.

Store cooled treats in an airtight container at room temperature or in the refrigerator.