Preheat your oven to 350°F (175°C) and lightly grease a 9x13 inch baking dish.
Peel, core, and slice the apples into ¼-inch thick pieces, ensuring all slices are of similar thickness for even cooking.
In a large mixing bowl, gently toss the apple slices with spiced rum, allowing the fruit to absorb the flavors.
Add the brown sugar, lemon juice, cinnamon, nutmeg, cloves, cornstarch, and salt to the apples, then toss until every slice is evenly coated.
In a separate bowl, combine the oats, flour, brown sugar, cinnamon, and salt for the topping.
Using your fingers or a pastry cutter, work the cold butter into the dry ingredients until you achieve a crumbly texture with pea-sized butter pieces throughout.
If using nuts, fold them into the topping mixture.
Transfer the apple mixture to your prepared baking dish, spreading it evenly.
Sprinkle the crisp topping uniformly over the apples, ensuring complete coverage.
Bake in the preheated oven for 45-50 minutes, or until the topping is golden brown and the filling is bubbling around the edges.
Allow the crisp to rest for 5-10 minutes before serving to let the filling set slightly.