Preheat your oven to 350°F (175°C) and prepare a loaf pan by greasing it or lining it with parchment paper.

In a large mixing bowl, mash the ripe bananas until smooth, then grate and add the zucchini after squeezing out excess moisture.

In another bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon.

Fold the dry ingredients into the banana-zucchini mixture gently without overmixing.

For the streusel, mix butter, brown sugar, granulated sugar, flour, and walnuts until crumbly.

Pour the batter into the loaf pan, smoothing the top, and sprinkle the walnut streusel over it.

Bake for 55 to 70 minutes, checking for doneness with a toothpick.

Cool in the pan for 15 minutes, then transfer to a wire rack to cool completely.