Preheat the oven: Preheat your oven to 350°F (175°C).
- Core the apples: Core the apples, removing the seeds, and set them in a baking dish.
- Make the streusel: In a small bowl, combine the rolled oats, chopped pecans, brown sugar, flour, cinnamon, nutmeg, and salt. Pour in the melted butter and vanilla extract, stirring until the mixture becomes crumbly.
- Assemble the apples: Spoon the streusel mixture generously over the apples, filling the centers and covering the tops.
- Add water: Pour about 1/4 cup of water into the bottom of the baking dish to help steam the apples as they bake.
- Bake: Cover the baking dish with foil and bake for 25 minutes. Remove the foil and bake for an additional 15-20 minutes, or until the apples are tender and the streusel is golden brown.
- Cool and serve: Let the baked apples cool slightly before serving. Enjoy warm with a scoop of vanilla ice cream or a drizzle of caramel sauce.