Heat the oven to 375°F (190°C).
Using a melon baller or small spoon, core the apples from the top, leaving the bottom 1/2 inch intact so the filling stays in. Make the opening about 1 1/2 inches wide.
Mix the oats, nuts, sugar, cinnamon, nutmeg and salt in a bowl. Add the butter and rub it in with your fingertips until the mixture clumps.
Stand the apples in a small baking dish and pack the filling into the cavities, mounding the extra on top.
Pour the water or cider into the dish. Bake for 35–45 minutes, until a knife slides easily into the apples and the topping is golden. If the topping browns too fast, cover loosely with foil.
Let cool for 10 minutes, then serve with some of the pan juices spooned over and a spoonful of yogurt or ice cream.