Preheat the oven to 350° F (180° C) and line a 9x5 inch loaf pan with parchment paper. Grate the zucchini, squeeze out the excess liquid, and set aside. Chop the walnuts and set aside. In a medium bowl, mash the bananas.
In another medium bowl, stir together the flour, brown sugar, baking powder, salt, cinnamon, nutmeg, and cardamom until well combined.
In a large bowl, whisk together the oil, eggs, and vanilla extract. Next, whisk in the mashed bananas. Add about half of the flour mixture and whisk until almost combined. Switch to a large spoon and add the remainder of the flour mixture, as well as the zucchini and walnuts, and stir until fully combined. Be careful not to overmix.
Pour the batter into the loaf pan and bake at 350° F (180° C) for about 50-60 minutes, or until a toothpick inserted into the middle comes out clean. Store at room temperature for about 3 days, or in the refrigerator for 4-5 days.
