Preheat the oven to 350°F. Prepare a 9x5 bread pan with nonstick spray and/or parchment paper, leaving a little overhang on the sides if you'd like to lift the loaf out easily.
In a medium mixing bowl, whisk together the flour, cinnamon, baking soda, baking powder, salt, and nutmeg. Set aside.
In a small bowl, stir together the cinnamon and sugar for the topping. Set aside.
In a large mixing bowl, mash the ripe banana well, then add the sugar, shredded zucchini, beaten egg, and lemon zest. Stir until everything is combined.
No ripe bananas? Bake unpeeled bananas on a lined baking sheet at 300°F for 15 to 40 minutes, until the skins turn black and the bananas are soft.
Add the dry ingredients to the wet ingredients and stir just until no dry flour remains and a thick batter forms. Be careful not to overmix, which can make the bread tough.
Spoon the batter into the prepared pan and spread it evenly. Sprinkle the cinnamon sugar mixture over the top for a crunchy topping.
Bake for 50 to 55 minutes, or until a toothpick inserted into the center comes out clean. Check the bread after 35 minutes, and if the top is browning too quickly, cover it loosely with foil.
Let the bread cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely before slicing.
Notes
This bread is dense and moist, closer to banana bread than typical zucchini bread, since the banana replaces the oil or butter.
Stir the dry ingredients in just until combined. Overmixing makes the bread tough.
Use very ripe bananas for the best sweetness. No ripe ones? Bake unpeeled bananas at 300°F for 15 to 40 minutes, until the skins turn black.
There's no need to peel or squeeze the zucchini. The skin softens as it bakes, and the natural moisture keeps the loaf tender.
Check the bread after 35 minutes. If the top is browning too much, cover it with foil to prevent burning. A crack down the middle is normal for quick breads.
This bread is delicious warm, but if you wrap it and let it sit overnight, the flavors deepen, and it's easier to slice.
Yellow squash can be used in place of zucchini. Orange zest works in place of lemon zest, and a flax egg (1 tablespoon ground flaxseed mixed with 3 tablespoons water) can replace the egg.
For fun variations, fold in ½ cup of chopped walnuts, pecans, chocolate chips, or cinnamon chips, or bake the batter as muffins.
Wrap leftovers in plastic wrap or store in a sealed container for 2 to 3 days at room temperature. It will keep covered in the refrigerator for up to one week, or in the freezer for up to three months.