Preheat oven to 375°F (190°C). Lightly grease an 8x8-inch or 2-quart ceramic baking dish with butter.

In a large mixing bowl, combine rolled oats, baking powder, cinnamon, nutmeg, and salt. Spread half of the oat mixture across the bottom of the dish.

Layer diced Honeycrisp apples, chopped pecans, and raisins over the oats, then scatter remaining oat mixture evenly on top.

In a measuring pitcher, whisk together milk, maple syrup, eggs, melted butter, and vanilla extract until frothy and well blended.

Pour the liquid custard slowly and evenly over the oats and apples, ensuring all dry spots are completely saturated. Let rest on the counter for 5 minutes to absorb.

Bake at 375°F (190°C) for 25 to 28 minutes until the top is golden brown and the center is tenderly set with no liquid pooling.

Cool for 5 minutes. Spoon into warm bowls, drizzle with cold milk or cream and a swirl of pure maple syrup, and serve immediately.