Preheat your oven: Set it to 350°F (175°C). Lightly grease an 8x8-inch baking dish or similar.
Mix the dry ingredients: In a large bowl, combine oats, baking powder, cinnamon, and salt.
Stir to evenly distribute the spices.
Whisk the wet ingredients: In a separate bowl, whisk milk, eggs, maple syrup, vanilla, and melted coconut oil or butter.
Combine and fold: Pour the wet mixture into the dry and stir until combined. Fold in the diced apples and any optional nuts or dried fruit.
Transfer to the dish: Pour the mixture into the prepared baking dish and spread evenly. Sprinkle a pinch of cinnamon on top if you like.
Bake: Bake for 35–40 minutes, until the center is set and the edges are golden.
A knife inserted in the center should come out mostly clean.
Rest and serve: Let it cool for 10 minutes to set. Slice and serve warm with a spoonful of yogurt or a splash of milk.