Preheat the oven to 350°F (175°C). Grease an 8x8-inch baking dish with butter or oil.
Peel (optional) and dice the apples into small, bite-size pieces.
You want about 2 cups.
In a large bowl, whisk together 2 cups rolled oats, 1 teaspoon baking powder, 1 1/2 teaspoons cinnamon, 1/4 teaspoon nutmeg (optional), and 1/2 teaspoon salt.
In a separate bowl, whisk 1 3/4 cups milk, 2 eggs, 1/3 cup maple syrup, 2 teaspoons vanilla, and 3 tablespoons melted butter or coconut oil.
Pour the wet ingredients into the dry and stir until combined. Fold in the diced apples and, if using, 1/3 cup chopped nuts and 1/3 cup raisins.
Transfer the mixture to the prepared baking dish and spread evenly. Sprinkle a little extra cinnamon on top if you like.
Bake for 35–40 minutes, until the center is set and the top is lightly golden.
The edges should be slightly crisp.
Cool for 10 minutes. Slice and serve warm with a splash of milk, a drizzle of maple syrup, or a dollop of yogurt.