Step 1: Preheat your oven to 350°F (175°C) for even baking.
Step 2: Grease your 9x9-inch baking dish with coconut oil or butter.
Step 3: In a large mixing bowl, combine rolled oats, half of the chopped walnuts, baking powder, and ground cinnamon. Stir until well mixed.
Step 4: In another bowl, whisk together maple syrup, almond milk, beaten egg, melted coconut oil or butter, and vanilla extract until smooth.
Step 5: Pour the wet mixture over the dry ingredients and stir until all oats are well coated. Gently fold in sliced bananas and mixed berries.
Step 6: Spread the oatmeal mixture in the baking dish. Sprinkle remaining walnuts on top. Bake for 30-35 minutes until golden brown.
Step 7: Let cool for a few minutes, cut into squares, and serve warm.
