Preheat the oven to 375°F (191°C).
In a bowl, mash together the butter, sugar, cinnamon, and nutmeg until well combined. Stir in the oats and your choice of raisins, dried cranberries, or nuts. Set aside.
Core the apples using a sharp paring knife and spoon. Cut around the core, then carefully scoop it out, removing any seeds.
Place the cored apples in an 8 or 9-inch baking dish.
Spoon the filling into each apple, packing it all the way to the top.
Pour warm water into the pan around the apples to prevent them from drying out.
Bake for 40-45 minutes, or until the apples are soft. For extra soft apples, bake a little longer.
Serve warm with salted caramel, whipped cream, or ice cream. Store leftovers in the refrigerator for up to 2 days.