Preheat the oven to 350 °F. Butter a 9" square pan.
In a large bowl, combine 10 oz old-fashioned rolled oats, 2 teaspoons baking powder, 1 teaspoon ground cinnamon, ½ teaspoon table salt, and ¼ teaspoon nutmeg.
In a large measuring cup, whisk together 12 oz whole milk, 4 oz unsalted butter (melted), 4 oz real maple syrup2 large eggs and 1 teaspoon vanilla extract.
Stir the liquid into the oat mixture until combined. Fold in 1 large apple (chopped) and 2 oz walnuts (chopped). Pour the batter into the prepared pan and smooth to an even layer. Sprinkle with 2 tablespoons granulated sugar for topping.
Bake until the center springs back when lightly pressed or a toothpick inserted into the center comes out clean, 30-35 minutes. Optionally, you can turn on the broiler for a minute or two after the oatmeal is done baking. The broiler will caramelize the sugar for an extra crunchy topping.
Cool 10 minutes before slicing.
Notes
This is delicious served warm or at room temperature as a snack. Keep leftovers refrigerated and microwave to reheat.