Preheat your oven to 180°C (fan-assisted) or 200°C (conventional). Lightly grease a 9x13 inch baking dish with a smear of butter or a spritz of oil. You'll know it's ready when you feel a gentle heat radiating from the oven door — no need to wait for a beep if you're short on time.

In a large mixing bowl, combine the rolled oats, baking powder, cinnamon, and salt. Stir them together until the cinnamon has dispersed evenly, turning the oats a pale, warm tan colour. You should smell the cinnamon as you stir.

In a separate smaller bowl, mash the bananas with a fork until they form a lumpy, thick purée. They should look like a soft, golden-brown paste with a few small chunks remaining — that texture adds lovely pockets of sweetness throughout the bake.

Add the eggs, milk, maple syrup, vanilla extract, and melted coconut oil to the mashed bananas. Whisk everything together until the mixture is a uniform, pale beige liquid. It should smell sweet and slightly floral from the vanilla, with no streaks of egg white visible.

Pour the wet mixture into the dry ingredients and fold together with a spatula. Stop as soon as the oats are fully coated — you'll see a thick, porridge-like batter that looks slightly glossy from the oil. Do not overmix, or the texture will become heavy and dense.

Gently fold in the blueberries. If you're using frozen blueberries, they'll bleed a little purple into the batter, creating beautiful marbled swirls. Fresh berries will stay more intact, popping with juice when you bite into them.

Pour the batter into the prepared baking dish and spread it into an even layer. The surface should look bumpy with berries and oats, not perfectly smooth. Sprinkle over the chopped nuts if using.

Bake for 28 to 32 minutes, or until the top is a deep golden brown and the edges are pulling away from the sides of the dish. The centre should feel firm but springy when you press it gently with your finger — like a firm sponge cake. A skewer inserted into the middle should come out clean, with no wet batter clinging to it.

Let it rest in the dish for a full 10 minutes before slicing. During this time, the steam redistributes and the oatmeal sets properly. You'll notice the top relaxing slightly and the sides becoming easier to cut.