Preheat oven to 350° F. Lightly grease a 9x5 inch loaf pan with nonstick spray.
In a large bowl, whisk together flour, baking soda, salt, nutmeg, cinnamon, and orange zest. Set aside.
In a separate bowl of a stand mixer, cream together butter and brown sugar. Stir in eggs and mashed bananas until well blended.
By hand, stir banana mixture into the flour mixture, until moistened.* Gently stir in walnuts just until incorporated. Take care not to over stir as this will result in a dry, dense loaf. Pour batter into prepared loaf pan.
Bake in preheated oven for 60 to 65 minutes, until a toothpick inserted into center of the loaf comes out clean. Let bread cool in pan for 10 minutes, then turn out onto a wire rack and let cool completely.
Notes
*It is okay if there are still streaks of flour at this point because you will be stirring in the walnuts, which will diminish the presence of the flour.