Core each apple, leaving the bottom of the apple intact (a melon baller works great for this). Alternately, you may cut the apples in half and scoop out the core creating a cavity for the filling. Set aside.
Combine mulling spice, salt, flour, nuts, and oats in a medium bowl. Cut the butter into small pieces and add to the mixture, using a fork to incorporate it. The mixture should hold together if you gather clumps and press together.
Divide filling between the prepared apples then place apples in baking dish. Pour about 3/4 cup of water in bottom of the dish.
Bake until the apples are soft and filling is browned, about 1 hour.
Serve warm with vanilla ice cream and caramel sauce.