Preheat the oven to 180°C/350°F and line muffin pans with cupcake liners.
Whisk the flour, rolled oats, baking powder, salt and cinnamon together in a large mixing bowl.
In a separate bowl, whisk together the bananas, eggs, brown sugar, vanilla and oil.
Pour the wet ingredients into the dry ingredients and mix until just combined.
Fold in the walnuts and seeds.
Transfer the batter to the prepared pans, filling each muffin cup.
Top the batter with a few teaspoons of crumble mixture.
Place the muffins pans in the preheated oven and allow to bake for 20-25 minutes until golden brown and a toothpick inserted comes out clean.
Remove from the oven and allow to cool completely before serving.
