Preheat the oven to 350°F. Generously grease a 9- by 5-inch loaf pan with a bit of olive oil.
As the oven heats, toast the nuts until they are fragrant but not dark, 5 to 10 minutes. Check them after 5 minutes. If they aren't ready, stir them and recheck every minute or two.
In a small bowl, whisk together the flours, salt, cinnamon, baking soda and baking powder.
In a large bowl, beat the egg until light. Add the sugar, mixing well. Stir in the mashed banana, sourdough discard, olive oil and vanilla.
Add the dry ingredients to the banana mixture and stir just until combined. Fold in the nuts.
Scrape the batter into the prepared pan and bake for 50 to 60 minutes, until a thin clean sharp knife inserted into the center comes out clean. Cool the loaf in the pan for 10 minutes before inverting onto a rack to cool completely. If necessary, run a knife along the edges of the pan to release the banana bread.
Notes
This banana bread is only slightly sweet. If you ordinarily reduce the amount of sugar in quick bread recipes, you may not want to reduce the sugar in this recipe by very much.