Place raisins and rum in a small pan over low heat. Bring just to a boil. Cover; remove from heat.
Preheat oven to 375°F. Butter a 9-by-13-inch baking dish. Line a baking sheet with foil. Make topping: mix oats, flour, sugar, cinnamon, ginger, nutmeg and salt. Stir in butter until crumbly. Toss in walnuts, if desired.
In a separate bowl, toss apples with sugar, flour and salt. Drain raisins; reserve 1 Tbsp. rum. Toss raisins with apple mixture and reserved rum. Spread apple mixture in baking dish. Sprinkle evenly with topping. Place baking dish on sheet and bake until filling bubbles and topping is golden brown, 55 to 60 minutes. Let cool on a wire rack. Serve warm with vanilla ice cream, if desired.