Preheat the oven to 350ºF.
In a large bowl, mash four of the ripe bananas. Mash until mostly smooth but some lumps remain.
Add the oil, 1/2 cup brown sugar, eggs, and vanilla. Mix well.
Add the salt, baking soda, 1 cup whole wheat flour, cinnamon, and rolled oats. mix to combine.
Separately, combine the blueberries and the 1 tbsp of flour and then add to the main mix.
Grease your pan by spraying it with nonstick spray or spread over some butter. Sprinkle about 1/2 the remaining brown sugar, shaking the pan to distribute it.
Pour in the batter and spread it out. Slice the remaining banana and arrange the slices as shown in the picture. Sprinkle with the left over brown sugar.
Bake in the preheated oven for 20-22 minutes, then optionally broil on the low setting for 3 minutes to caramelize the sugar on top. Enjoy!