Preheat and prep the baking dish: Preheat oven to 375°F (190°C). Grease an 8x8-inch or 9x9-inch ceramic casserole baking dish lightly with butter or coconut oil.

Layer dry ingredients: In a large bowl, whisk together rolled oats, baking powder, ground cinnamon, nutmeg, and sea salt. Spread half of the spiced oat mixture across the bottom of the prepared dish. Scatter half of the diced apples and half of the chopped pecans over the oats. Top with the remaining oats, followed by the rest of the diced apples and pecans.

Whisk the custard: In a medium mixing bowl or large glass measuring jug, whisk together milk, beaten eggs, 1/3 cup maple syrup, melted butter, and vanilla extract until completely smooth and emulsified.

Pour and soak: Slowly pour the liquid custard evenly over the layered oats and apples. Gently tap the baking dish on the countertop twice so the custard seeps all the way down into the base of the oats. Let rest on the counter for 5 minutes before baking so the oats start absorbing the liquid.

Bake: Bake at 375°F (190°C) for 35 to 40 minutes until the top is set, deep golden-brown, and a toothpick inserted in the center comes out clean with no liquid custard.

Glaze and serve: Remove from the oven and let cool for 5 minutes. Drizzle the top with the remaining 2 tablespoons of warm pure maple syrup. Slice into hearty squares and serve warm with a splash of cream or cold milk!