Preheat the oven to 325°F. Grease an 8 or 9-inch baking dish with butter.
In a medium bowl, combine the oats, brown sugar, ½ cup of the nuts, raisins, baking powder, cinnamon and salt. Mix well.
In another bowl, break up the eggs with a whisk; then whisk in the milk and vanilla until well combined.
Add the milk mixture to the oat mixture, along with the melted butter.
Scatter the apples evenly on the bottom of the prepared baking dish. Pour the oatmeal mixture over top and spread evenly. Sprinkle remaining ½ cup nuts on top. Bake for 40-45 minutes, until the top is golden and the oats are set. Serve warm or at room temperature.
Freezer-Friendly Instructions: This dish can be frozen after baking, tightly covered, for up to 3 months. When you’re ready to serve it, defrost in the refrigerator for 24 hours then reheat it, covered with foil, in a 325°F oven until hot.