Thaw your bananas in a bowl—if you need to speed things up, cover them with warm water and they’ll thaw quickly.
When you’re ready to bake, preheat the oven to 350F.
Cut the stem ends off the bananas and cut them into chunks—I like doing this with scissors directly over the bowl of the food processor, slicing them in. Put the brown sugar, butter or oil, egg and vanilla into the food processor as well and pulse until well blended and relatively smooth, scraping down the side of the bowl. Don’t worry there are some black flecks of skin in it.
Scrape into a bowl and add the flour, cinnamon (if you’re using it), baking soda and salt. Stir a few strokes, then add the walnuts or pecans and stir just until combined. Scrape into a parchment-lined or greased 4x8-inch loaf pan and smooth the top. Sprinkle with demerara or turbinado sugar, if you like.
Bake for about an hour, or until golden, domed and springy to the touch. Cool in the pan for about 20 minutes, then take it out and cool on a wire rack, if you have one.
Makes 1 loaf, and about 10 slices.