Preheat the oven to 375° F and spray an 8x8 inch square baking dish with cooking spray.
Mix dry ingredients: In a large bowl, mix together the oats, baking powder, cinnamon, and salt.
Add in wet ingredients: Add in the milk, sugar, flaxseed, coconut oil, vanilla and bananas. Stir to combine and then add in any mix-ins that you're using.
Carefully pour oatmeal mixture into the prepared baking dish.
Scatter the remaining mix-ins across the top.
Bake: Bake for 35 to 40 minutes, until the top is nicely golden. Remove from the oven and let cool for a few minutes. Portion and serve. Drizzle each portion with maple syrup if you want it a bit sweeter.
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 oven (or toaster oven) to 350°F and bake for 5-10 minutes or reheat in the microwave for 1 minute.