Preheat oven to 350°F (175°C). Line a 12-cup muffin tin or grease lightly.
In a large bowl, whisk mashed bananas, applesauce, eggs, brown sugar, and vanilla until smooth.
Add oats, flour, baking soda, baking powder, and salt. Mix until just combined.
Gently fold in berries and walnuts.
Divide batter evenly into muffin cups, filling almost to the top.
Bake for 15–18 minutes until set and lightly golden.
Cool for 5 minutes, then transfer to a wire rack.