Preheat oven to 375°F (190°C). Grease an 8x8-inch or 9-inch round ceramic casserole dish with melted butter or coconut oil.
In a large mixing bowl, combine the rolled oats, ground cinnamon, nutmeg, baking powder, sea salt, and half of the chopped pecans. Stir until evenly distributed.
In a medium bowl, whisk together the eggs, almond milk, pure maple syrup, melted butter, and vanilla extract until completely smooth.
Fold the diced Honeycrisp apples into the dry oat mixture, then pour the wet liquid mixture over the oats. Stir gently with a wooden spoon until the oats are thoroughly saturated.
Pour the mixture into the prepared casserole dish. Arrange thin apple slices across the center and scatter the remaining chopped pecans on top.
Bake for 32 to 36 minutes until the top is golden brown and the center is softly set. Rest for 5 minutes, slice into hearty squares, and serve warm with a drizzle of maple syrup and a splash of milk.
