Preheat your oven to 350 degrees Fahrenheit and prepare a 12-cup muffin tin with paper liners (or grease very well with butter or cooking spray if you prefer not to use paper liners). Mash the bananas in a large bowl and mix in the sugar and egg with a wooden spoon until well blended.

Add the vegetable oil and mix well. Add the flour, baking powder, baking soda, salt, cinnamon and nutmeg, along with 3/4 cup of the chopped walnuts. Adding the walnuts with the dry ingredients helps prevent over-mixing.

Mix everything together just until the flour disappears and the walnuts are distributed evenly throughout the batter. In a small bowl, combine the remaining 3/4 cup walnuts, brown sugar and melted butter and mix until the walnuts are coated.

Spoon the muffin batter evenly into the 12 muffin cups and spoon a bit of the coated walnuts onto the top of each one, making sure the topping is well distributed onto the top of each muffin (not in one big clump or it might sink into the muffin as it bakes!).

Bake for about 20-23 minutes, or until the muffins are golden brown on top and a toothpick inserted into the center of one of the muffins comes out clean. Let the muffins cool in their tins for a few minutes before removing them to a wire rack to cool completely.