Preheat Oven: Set the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease with butter or non-stick spray.
Mash Bananas: In a large bowl, mash the ripe bananas with a fork until smooth.
Mix Wet Ingredients: To the bananas, add the sugar, vegetable oil, and egg. Mix well until all the ingredients are fully combined.
Sift Dry Ingredients: In another bowl, sift together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg if using.
Combine Mixtures: Gradually add the dry ingredients to the wet ingredients, stirring until just combined. It's crucial not to overmix to avoid dense muffins.
Add Walnuts: Fold in the chopped walnuts, ensuring they are evenly distributed throughout the batter.
Fill Muffin Cups: Spoon the batter into the prepared muffin cups, filling each about three-quarters full.
Bake: Place the muffin tin in the oven and bake for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Cool: Allow the muffins to cool in the pan for 5 minutes, then remove them and place on a wire rack to cool completely.