Preheat oven to 350. Grease a 8.x8 or 9.x9 baking dish. Slice peach and bananas.
Mix dry ingredients (oats, cinnamon, baking powder, flax seed, and salt) in a large bowl.
Mix wet ingredients (eggs, milk, vanilla extract, and sugar if using) separately then incorporate into dry ingredients. Finally add in fruit, leaving one banana out for the topping. Pour batter into dish, topping it with the final banana and an extra pinch of cinnamon.
Bake for 35-40 minutes until oatmeal is tender, or until top is golden and liquid is absorbed.
Let cool for 15-20 minutes before cutting. Store in the fridge for up to a week!