Adjust the oven rack to the middle position and preheat the oven to 325 degrees.

Spread the nuts on a baking sheet and toast in the oven until lightly browned, about 8 to 10 minutes. Shake the pan halfway through to ensure that the nuts toast evenly. Cool, chop coarsely, and set aside.

Turn the oven up to 350 degrees.

In a medium bowl, whisk the banana puree, eggs, and vanilla extract to combine.

In the bowl of an electric mixer fitted with the paddle attachment, cream the butter, baking soda, baking powder, salt, cinnamon, nutmeg, cloves, and poppy seeds on low, 2 to 3 minutes, until softened. Add the sugars and turn the mixer up to medium, mixing another 3 to 4 minutes until fluffy, scraping down the sides of the bowl as needed.

Add the flour and the banana mixture alternately in 3 batches, beginning with the flour and mixing until just combined.

Remove the bowl from the mixer and fold in the nuts.

Pour the batter into the prepared loaf pan to just below the rim.

Cut two ¼-inch-thick strips from the remaining banana, slicing down the entire length. Arrange the two C shapes on top of the loaf, staggered, with the two ends slightly interlocking with each other in the center. Sprinkle about one teaspoon of granulated sugar over the surface.

Bake for 50 to 60 minutes, until nicely browned and firm to the touch.