Preheat oven to 350°F (175°C) and grease a 9x5 loaf pan. Mash bananas in a large bowl, then whisk in melted butter, both sugars, eggs, and vanilla until combined.

Sprinkle flour, baking soda, baking powder, salt, cinnamon, and nutmeg over the wet mixture. Fold gently with a spatula until just combined, then fold in shredded apple and nuts if using.

Scrape batter into prepared pan and smooth the top. Arrange thin apple slices down the center and sprinkle with cinnamon sugar. Bake 55 to 65 minutes until a toothpick comes out with a few moist crumbs.

Cool in the pan for 15 minutes, then transfer to a rack to cool completely. Slice with a serrated knife and serve.