In a large bowl, combine the oats and milk and let them soak for 15–20 minutes.
Preheat the oven to 350°F and lightly grease or line a baking dish with parchment paper.
In a separate bowl, mash the banana until smooth.
Add the eggs and plain yogurt to the mashed banana and mix well.
Stir the banana mixture into the soaked oats until combined.
Mix in the baking powder until evenly distributed.
Fold in the diced apple, then gently add the blueberries.
Pour the mixture into the prepared baking dish and spread it evenly.
Bake for 35–40 minutes, until the top is golden and the centre is set.
Let it cool for about 10 minutes before slicing and serve warm with extra yogurt, honey, maple syrup or more berries if desired.