Preheat your oven to 350°F (175°C) and grease a baking dish.
In a large mixing bowl, whisk together eggs, milk, maple syrup (or honey), and vanilla extract until well combined.
Stir in the rolled oats, baking powder, cinnamon, and salt into your egg mixture until evenly distributed.
Gently fold in the diced apple and optional chopped walnuts or pecans.
Pour the mixture into the greased baking dish and spread it evenly. Bake for about 30-35 minutes until golden and set.
Let it cool for about five minutes before serving warm.