Preheat the oven to 325°F. Grease a 9-inch baking dish with butter; set aside.
In a medium bowl, combine the oats, ½ cup pecans, baking powder, 2 teaspoons cinnamon, nutmeg, and salt. Mix well.
In another bowl, break up the eggs with a whisk; then whisk in the milk, maple syrup, and vanilla until well combined.
Add the milk mixture to the oat mixture, along with the melted butter. Stir well to combine.
Combine the diced apples and remaining pecans in a bowl. Toss with the remaining cinnamon, then scatter the apples evenly on the bottom of the prepared baking dish.
Pour the oatmeal mixture over top and spread evenly. Garnish top with thinly sliced apples if desired. Bake for 40-45 minutes, until the top is golden and the oats are set. Serve warm or at room temperature.
*Nutritional information is an estimate. Please consult a doctor or nutritionist for the most accurate nutritional info.