I always start by preheating my oven to 350°F so it's fully heated by the time my batter is ready. While it warms up, I grease and lightly flour a 9x5 inch loaf pan so the bread releases cleanly later. Skipping this step is one of the easiest ways to end up with a stuck loaf.
In a medium bowl, I whisk together the flour, pumpkin pie spice, cinnamon, baking soda, baking powder, and salt. Whisking instead of just stirring helps distribute the leavening agents evenly throughout the flour. This little step makes a noticeable difference in how evenly the bread rises.
In a separate large bowl, I whisk the oil, both sugars, eggs, and vanilla extract until the mixture looks smooth with no lumps. This usually takes about a minute of steady whisking. Getting this base smooth now means less mixing later, which keeps the bread tender.
Next, I stir in the mashed bananas and pumpkin puree until they're fully incorporated into the wet mixture. The batter will turn a beautiful golden-orange color at this point. This is where the loaf really starts to smell like fall.
I gently fold the dry mixture into the wet ingredients using a rubber spatula, mixing just until no streaks of flour remain. If I'm adding walnuts, I fold them in at this stage too. Overmixing here is the number one thing I try to avoid, since it can make the bread tough.
I pour the batter into my prepared pan and smooth the top with my spatula before sliding it into the oven. It bakes for about 50 to 60 minutes, until a toothpick inserted in the center comes out clean and the top feels firm. Once it's ready, I let it cool in the pan for about 10 minutes before moving it to a wire rack.