In a small bowl, pour rum over raisins and allow the raisins to plump for 30 minutes.
Grease 4 individual ramekins with butter.
Preheat oven to 350˚F (180˚C).
In a medium bowl, mix together the apples, 1 teaspoon cinnamon, nutmeg, 2 tablespoons brown sugar, lemon juice.
Add the soaked raisins and the rum. Stir to coat.
Add 1/4 apple mixture to each ramekin.
In a small bowl, add chopped walnuts, rolled oats, 1 teaspoons cinnamon, 2 tablespoons brown sugar and a pinch of salt and mix to incorporate.
Use your fingers to mash the butter into the mixture, creating small clumps.
Divide the mixture on top of each of the 4 ramekins, pressing down lightly on the tops.
Bake in the 350˚F (180˚C) oven for 40-45 minutes and tops are nicely browned.
Allow to cool for 20 minutes.
Serve topped with vanilla ice cream or freshly whipped cream.