Preheat the oven to 350℉ and grease a 9×5-inch pan and line with a parchment sling if desired.
In a large bowl, stir together the 1 ½ cups mashed ripe bananas, 1 cup shredded zucchini, ½ cup unsalted butter, and 2 large eggs.
In a medium bowl, whisk together 2 ½ cups oat flour, ¾ cup brown sugar, 2 teaspoons baking powder, 1 teaspoon ground ginger, and ½ teaspoon fine sea salt. Stir the dry mix into the wet ingredients and fold in the 1 cup walnuts if using.
Pour the batter into the prepared pan and smooth the top. Let batter rest for 10-15 minutes, then bake for 55-60 minutes until golden brown a cake tester comes out clean.
Cool in the pan for 15 minutes, then gently remove from the pan and let cool completely on a wire rack.
Notes
Use frozen bananas: Save ripe bananas (skin on or off) in a freezer-safe bag in the freezer. When you're ready to make this recipe, take out as many bananas as you need, let them defrost in a bowl, and drain away any liquid before mashing them up for the recipe.
Use another flour: You can use a gluten-free flour blend instead of the oat flour by weight. Or try another single-origin gluten-free flour, like sorghum flour or brown rice flour, by weight. Or, swap 2 cups of all-purpose flour or whole wheat pastry flour if you don't need to keep gluten-free.
Try other spices. Instead of ginger, use ground cinnamon or cardamom. Add a pinch of nutmeg for a slightly different flavor note. Or, use pumpkin pie spice!
Add vanilla. Whisk in 2 teaspoons of pure vanilla extract or vanilla bean paste when combining the wet ingredients.
Variations
Chocolate - I love regular or mini semisweet chocolate chips in this bread to make it a little extra!
Carrots - Swap the zucchini for shredded carrots, or stir in ½ cup of carrots in addition to the zucchini.
Dairy-free - Easily make this dairy-free with your choice of ½ cup of oil instead of the butter.
Applesauce - Reduce the amount of butter/oil by doing a combination of ¼ cup of unsweetened applesauce and ¼ cup of butter or oil.
Lower sugar - Reduce the amount of sugar to ½ cup, especially if your bananas are very sweet.