Preheat the oven to 350°F (175°C) and lightly grease a 9×5 inch loaf pan. Mash the bananas in a mixing bowl until mostly smooth.
Add the egg, maple syrup, Greek yogurt, melted coconut oil, and vanilla extract. Whisk until the mixture becomes smooth and slightly creamy.
In the same bowl, add whole wheat flour, rolled oats, baking soda, cinnamon, and salt. Stir gently until just combined and a thick batter forms.
Fold in the chopped walnuts if using. The batter should look slightly lumpy but evenly mixed.
Pour the batter into the prepared loaf pan and smooth the top with a spatula so it bakes evenly.
Bake for 45–50 minutes, or until the top is golden and a toothpick inserted in the center comes out clean.
Allow the banana bread to cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely before slicing.
