Preheat the oven to 350ºF and grease a loaf pan. Set aside.
Add your peeled bananas to a small bowl, and mash with a fork. You'll need approximately 2 cups of mashed bananas, which is equal to 2 very large bananas or 3 medium bananas.
In a medium sized bowl, add the flour, baking soda, salt, and cinnamon. Whisk well to combine and set aside.
Add the softened butter to the bowl of a stand mixer fitted with the paddle attachment OR a large bowl with a hand mixer.
Add the sugar to the butter, and turn the mixer onto medium speed. Beat the butter and sugar mixture until fully creamed, approximately 2 minutes. Stop the mixer and scrape down the sides of the bowl at least once during this time to ensure everything is uniform.
Add the eggs and vanilla extract, and beat until fully incorporated. Scrape down the sides of the bowl.
Add the sourdough discard (or active starter) to the bowl and mix just until combined.
Then, add the mashed banana and mix again.
Remove the bowl from the stand mixer and add in the combined dry ingredients. Fold them in with a silicone spatula, until all the flour has been moistened. Use a gentle hand, as you don't want to overwork the batter.
Add in the chopped walnuts or pecans and fold them in just until they are evenly distributed.
Pour the banana bread batter into the prepared loaf pan, and spread it in an even layer. Sprinkle with coarse sugar, if desired.
Bake in the preheated oven for 50-75 minutes, rotating the pan halfway through baking. The loaf is done when a toothpick comes out clean when inserted into the center. Keep in mind you may see some mashed banana, but you should not see batter.
Remove the banana bread from the oven and allow to cool in the loaf pan for 30-40 minutes.
Slide a butter knife around the edges of the loaf pan, and invert the loaf pan with your hand underneath to catch the loaf. Gently coax the loaf out and place it on a cooling rack. Allow to cool for at least 30 minutes before slicing, although it's best to allow it to cool to room temperature.
Slice and serve with butter, or on its own. Enjoy!