Preheat the oven to 375°F.
Spray an 8-inch square baking dish with cooking spray.
In a large bowl, mix together the oats, baking powder, cinnamon, and salt.
Add in the milk, maple syrup, flaxseed, peanut butter, vanilla and 2 sliced bananas. Stir well to combine.
Carefully pour oatmeal mixture into the prepared baking dish and top with banana slices.
Bake for 30 to 35 minutes, until the top is nicely golden. Remove from the oven and let cool for a few minutes. Portion and serve. Drizzle additional peanut butter on top and serve with fresh banana slices.
For storage: store in the refrigerator in an airtight container for up to 4 days.
To reheat: To reheat the whole baked oatmeal, cover with foil and reheat in a 350°F oven for about 20 minutes. For individual portions, set toaster oven to 350°F and bake for 5-10 minutes or reheat in the microwave for 1 minute.