Preheat oven to 350 degrees F. Grease a 9 x 13-inch baking dish with nonstick spray.
In a small or medium bowl, combine the oats, baking powder, cinnamon, and salt. Stir until combined and set aside.
In a large bowl, whisk together the egg, almond milk, melted coconut oil, pure maple syrup, vanilla extract, mashed banana, and melted honey. Whisk until just combined.
Add the dry ingredients to the wet ingredients and mix until combined. Do not overmix. Let sit for 10 minutes so the oats can soak up some of the liquid mixture.
Gently fold in 1 cup of blueberries and add some to the bottom of the baking dish. Sprinkle more blueberries on top; optional. Bake for 35-40 minutes just until the oatmeal is set and should be a light golden brown color on top. Let cool for a few minutes then cut into squares, serve and enjoy!
Top with maple syrup, whipped cream, chopped nuts, fresh fruit, or powdered sugar. Optional.