Preheat the oven to 375°F/190°C/Gas Mark 5, no fan (if you have a convection oven, reduce the temperature by about 20° or follow the manufacturer's instructions).
Prepare a 9-inch (23 cm) round baking dish (I used a pie dish) or a 8×8-inch square (20x20cm) baking dish (or similar) and grease it with 1 teaspoon of coconut oil.
Mash the bananas with a fork.
Add all the ingredients for the oatmeal except the oats into a medium bowl and whisk until very well combined. Next, whisk in the oats.
Pour the oatmeal batter into the baking dish.
Top with walnuts and blueberries.
Bake for about 30 minutes or until the oatmeal is set but still creamy (not dry). The baking time may vary depending on the oven.
Leave to set and cool for about 5 minutes, then serve (I like it most with plain yogurt and maple syrup).