Preheat the oven to 350°F/176°C. Arrange a rack in the bottom third of the oven. Line a 9x5-inch (23x13 cm) loaf pan with parchment paper, letting the excess hang over the long sides to form a sling.
Pour the aquafaba into a medium bowl. Using a handheld electric mixer, whip the aquafaba on medium speed for 45-60 seconds until uniformly foamy.
Wipe out the bowl used for the aquafaba. Add the three bananas to the bowl and mash with a fork until smooth and no lumps remain (or, for a more rustic banana bread, leave a few chunks in). FYI: the 3 mashed bananas should weigh around 300g.
Stir the lemon juice into the oat milk and set aside. This is the vegan "buttermilk."
In a large mixing bowl, add the sugar and oil. Mix using the electric mixer or a whisk until well combined.
Add the whipped aquafaba and mix until well incorporated. Add in the vegan "buttermilk" and vanilla and mix until smooth. Add the mashed bananas and mix until well incorporated.
Add the flour, salt, baking soda, and cinnamon to the wet ingredients. Switch to using a silicone spatula (if you don’t have one, use a large wooden spoon). Gently stir until the ingredients are just barely combined. It’s okay if you can see a few traces of flour.
Pour the batter into the prepared loaf pan and smooth out the top of the batter using a spatula or spoon.
For the optional decoration, keep the peel on the reserved banana and slice it in half, lengthwise; then peel the banana. Sprinkle a little coconut sugar or brown sugar on the cut/interior sides of the banana. Gently arrange the bananas on top the batter, sugared side side up.NOTE: This might create a dome in the middle of the bananas, where the batter bunches up in the middle. Use a spoon to move some of the batter around the pan so that the bread doesn’t bake unevenly.
Bake in the preheated oven for 45 to 55 minutes, or until a toothpick inserted into the center top (insert the toothpick at a slight angle) comes out mostly clean. Transfer the pan to a wire rack and cool for 10 minutes. Then remove the bread from the pan and cool on the rack for at least 20-30 minutes before slicing (the bread is still cooking and setting up, so don't slice too early).
To store, place completely cooled banana bread in an airtight container or wrap tightly in plastic and leave on the counter for 2-4 days. If you don't include the decorative banana on top, it will last longer. You can also freeze the banana bread. If you freeze the whole loaf, allow it to defrost on the countertop for 2-3 hours. If you freeze individual slices, defrost on the countertop for 20-30 minutes.