Start by melting butter in a small saucepan over medium heat. Continue to cook the butter, swirling the saucepan occasionally, until it turns golden brown and develops a nutty aroma. This typically takes about 5-7 minutes. Once the butter has turned a rich brown colour and smells nutty, remove it from the heat immediately to prevent it from burning and allow it to cool.
In a large mixing bowl, whisk together the eggs, maple syrup, milk and vanilla. Add the oats, cinnamon and baking powder. Stir to combine, then add the diced apple and finally the nut brown butter. Sprinkle with flaked almonds or chopped nuts, if using.
Grease a small tin, roughly 20cm x 20cm, then pour in the batter. Cover and refrigerate overnight. You can also go ahead and bake it straight away, if you wish.
The following morning, preheat the oven to 180*C. Bake for 35-40 minutes or until golden brown. We love to serve this warm out of the oven with a dollop of yoghurt, fresh berries and ground seeds.