Preheat the oven to 350°F. Spray a 9x13-inch baking dish with cooking spray. Mash the banana until no clumps remain. Optional: Lightly toast the nuts in a dry skillet over medium heat until fragrant, then set aside in a bowl to cool.
In a large mixing bowl, add the oats, baking powder, cinnamon, and salt. Whisk well to combine.
In a separate mixing bowl, add the Greek yogurt, milk, mashed banana, eggs, egg white, brown sugar or maple syrup, and vanilla. Whisk well to combine.
Add the wet ingredients to the bowl with the dry ingredients and mix well. Gently fold in the chopped nuts.
Pour the mixture into the prepared baking dish and let the batter rest for 5 minutes to allow the oats to soak in some of the moisture. Bake in the preheated oven for 30 minutes.
Let the baked oatmeal cool for 10 to 15 minutes before slicing into 12 squares. Sprinkle with extra nuts and a dollop of whipped cream before serving, or add a pat of butter, drizzle with maple syrup, top with a few slices of banana, and dust with cinnamon for the perfect finishing touch.