Preheat your oven to 350 degrees.
Butter a loaf pan, set aside.
In a small mixing bowl whisk together the flour, baking powder, baking soda, salt and cinnamon. Set aside.
In another a small mixing bowl combine the bananas and lemon juice. Mash with a fork until smooth. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment beat together the butter and sugar on high speed until fluffy, about 5 minutes.
Add in the eggs one at a time. Add in the bananas and mix until fully incorporated. Lower the speed to low and add in the dry ingredients. Mix until just combined. Do not over mix.
Fold in the walnuts and pour into the prepared pan. Bake for 1 hour or until an inserted toothpick comes out clean.