Preheat the oven to 370 degrees F. While the oven heats up, line a muffin tin with liners.
In a large bowl combine, whisk together 1.5 cups of flour with baking soda, baking powder, and salt.
In a medium bowl, combine the mashed bananas, sugar, egg, and melted butter. Whisk well until thoroughly combined.
Slowly and gradually add the banana mixture to the flour mixture while stirring it, just until it's lightly mixed. Gently fold in the blueberries and the walnuts.
In a small bowl, combine the coconut palm sugar, 1 tablespoon of flour, ground cinnamon, and rolled oats.
With a fork, cut the 1/2 tablespoon of butter into the mixture until it's slightly crumbly.
Evenly spoon the muffin batter into muffin papers. Sprinkle the sugar topping over the tops of the muffins.
Bake for 18-20 minutes, or until a toothpick or cake tester is inserted into the center and comes out clean.
Once done, remove, set on a cooling rack, and let cool for 5-10 minutes.