Preheat Oven & Prepare Pan: Preheat your oven to 350°F (175°C). Line an 8x8 inch baking dish with parchment paper to prevent sticking.
Prepare Oat Flour: Blend the rolled oats in a blender until they reach a flour-like consistency. Measure after blending to ensure you have the correct amount.
Mix Wet Ingredients: In a large mixing bowl, combine the mashed bananas, unsweetened applesauce, maple syrup, and vanilla extract. Stir until well combined.
Combine Dry Ingredients: In another bowl, mix together the oat flour, baking soda, baking powder, cinnamon (if using), and salt.
Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Avoid overmixing.
Fold in Berries and Nuts: Gently fold in the mixed berries (leave ¼ cup for decoration, optional) and optional nuts into the batter. If using frozen berries, there's no need to thaw them first.
Bake: Pour the batter into the prepared baking dish and smooth the top with a spatula. Add the rest of th reserved berries and the extra banana for decoration. Sprinkle shredded coconut on top (optional). Bake for 35-45 minutes*, or until a toothpick inserted into the center comes out clean and the edges are slightly golden brown.
Cool & Serve: Allow the cake to cool in the pan for at least 10 minutes before transferring it to a wire rack to cool completely. Cut into squares and serve.
Enjoy: Your Berry Banana Oatmeal Cake is ready to enjoy! Serve as is or with fresh berries, a dollop of vegan yogurt or a drizzle of maple syrup for extra sweetness.