Preheat oven to 350° F, coat an 8-muffin pan with butter.
Sift flour, cinnamon, baking soda, salt, nutmeg, and baking powder into a medium sized bowl. Using a fine sieve, working over the bowl, toss blueberries with 2 teaspoons of the flour mixture to coat. Set blueberries and flour mixture aside.
In a medium bowl, using a whisk, beat egg until frothy.
Add sugar and whisk until smooth.
Add lemon zest, mashed bananas and vegetable oil, whisk until combined.
Add flour mixture 1/3 at a time, whisk just until combined, do not over mix.
Using a rubber spatula fold in the blueberries and the walnuts, just until combined.
Bake, rotating loaf pan halfway through, until golden brown and a toothpick inserted in the centre comes out clean, about 55 to 60 minutes or if using small loaf pans, 25 to 30 minutes.