Preheat the oven to 350°F (180°C), 160°C fan, or Gas Mark 4.

In a large mixing bowl, combine the room-temperature butter, cooking oil, granulated sugar, and brown sugar.

Beat with a hand mixer or stand mixer until light and creamy.

Mash the bananas thoroughly.

Add the mashed bananas, eggs, and vanilla extract to the butter mixture.

Beat until smooth and well combined.

In a separate bowl, whisk together the flour, baking soda, salt, nutmeg, and cinnamon.

Add the dry ingredients to the wet ingredients and gently fold until just combined.

Avoid overmixing to keep the homemade banana bread tender.

Fold in the chopped walnuts if using.

Grease an 8.5 x 4.5-inch loaf pan with butter or oil.

Transfer the batter into the prepared pan and smooth the top.

Bake for approximately 60 minutes or until the loaf is golden brown and a toothpick inserted into the center comes out clean.

Let the bread cool in the pan for 5 minutes.

Remove from the pan and place on a wire rack to cool completely before slicing and serving.