Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment paper.
In a large bowl, whisk together mashed banana, grated zucchini, eggs, oil, brown sugar, granulated sugar, and vanilla until smooth.
In another bowl, whisk flour, baking soda, baking powder, salt, cinnamon, and nutmeg.
Add dry ingredients to wet ingredients and stir until just combined (don’t overmix!). Fold in nuts if using.
Pour batter into the prepared loaf pan. Smooth the top.
Bake for 50–55 minutes, or until a toothpick inserted in the center comes out clean.
Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely. Slice and enjoy!