Preheat oven to 350°F. Spray a 9×9-inch baking dish with nonstick cooking spray.
In a small bowl, toss the diced apples with ½ teaspoon cinnamon and 1 teaspoon maple syrup until evenly coated. Set aside.
Measure out about ½ cup of the coated apples and reserve for topping. Set the remaining apples aside to mix into the batter.
In a medium bowl, whisk together the oats, baking powder, salt, remaining cinnamon, and nutmeg.
In a large bowl, whisk the eggs until well combined. Add the milk, melted butter, remaining maple syrup, and vanilla extract. Whisk until smooth.
Add the oat mixture to the wet ingredients and stir until combined. Fold in the walnuts and the apples (excluding the reserved apples).
Pour the mixture into the prepared baking dish and spread into an even layer. Evenly scatter the reserved apples over the top.
Bake for 45 to 50 minutes, or until the center is set and the top is lightly golden.
Let cool for 5 to 10 minutes before serving.