Preheat the oven to 350°F. Thoroughly coat the bottom and sides of a loaf pan with butter or cooking spray to help prevent the bread from sticking.

Mash the ripe bananas until smooth. Grate the zucchini, then wrap it in a clean kitchen towel or several layers of paper towel and gently squeeze away the surplus moisture. Chop the pecans or walnuts when including them.

In a medium mixing bowl, whisk together the whole-wheat flour, all-purpose flour, milled flaxseed, baking powder, baking soda, pumpkin pie spice, and salt. Continue whisking until the leavening agents and seasonings are evenly dispersed, then set the bowl aside.

In a large bowl, combine the mashed banana, pumpkin purée, Greek yogurt, packed brown sugar, neutral oil, and room-temperature egg. Whisk until the mixture appears smooth and evenly blended.

Add the dry mixture to the wet ingredients. Using a spatula or wooden spoon, fold the batter only until the visible streaks of flour disappear. A few small lumps may remain; avoid excessive mixing, as it can produce a dense or tough loaf.

Gently fold the drained zucchini and chopped nuts into the batter until they are distributed throughout.

Transfer the batter to the prepared loaf pan and lightly smooth the surface. Bake at 350°F for 60–75 minutes. The loaf is ready when a toothpick inserted into the center emerges clean or carries only a few moist crumbs.

When the top begins browning too rapidly, loosely cover the loaf with aluminum foil at approximately the 40-minute point and continue baking until the center is fully set.

Remove the bread from the oven and leave it in the pan for 10 minutes. This brief resting period allows the loaf to firm sufficiently before removal.

Turn the bread out onto a wire rack and allow it to cool completely. Cooling it on a rack permits air to circulate beneath the loaf and helps prevent condensation from softening the bottom.