1. Preheat oven to 325°. Line a 9- by 13-inch baking dish with parchment paper.
2. Combine the oats, brown sugar, walnuts, raisins, cinnamon, baking powder and salt in a medium bowl. Mix well.
3. Combine the milk, vanilla and egg in another bowl and blend well. Add the liquid mixture to the oat mixture, along with the melted butter. Mix well with a large spoon. The mixture will be very liquid.
4. Using a paring knife, cut off the top part of each apple with the stem and reserve the tops. Push a paring knife through the apple and cut around the core, leaving the bottom ½ inch of each apple intact. With a small spoon, dig out the remaining core. Make the holes about 1 inch wide.
5. Place the apples on the prepared baking dish. Spoon the oatmeal mixture into the apple cavities, filling them to the top. If you have leftover oat mixture you can just bake it in a little dish. Bake until the tops of the apples are golden and the oats are set, about 40 minutes. Serve warm or at room temperature. Store leftover apples covered in the refrigerator for up to 3 days.