Preheat your oven to 350°F (175°C). Lightly grease a 9x9-inch baking dish or any similar-sized oven-safe dish and set aside.
In a large mixing bowl, combine the sliced apples with granulated sugar, brown sugar, cinnamon, nutmeg, allspice, lemon juice, and cornstarch. Toss everything together until the apple slices are thoroughly coated, then spread the mixture evenly into the prepared baking dish.
In a separate medium bowl, whisk together the oats, flour, brown sugar, granulated sugar, cinnamon, and salt. Add the cold butter cubes and use a pastry cutter or your fingertips to work the butter into the dry ingredients until the mixture forms coarse, crumb-like pieces. Stir in chopped nuts at this stage if you’re including them.
Evenly distribute the oat topping over the apples in the baking dish, covering them completely.
Place the dish in the preheated oven and bake for 35-40 minutes, or until the topping turns golden brown and the apples are tender when pierced with a fork.
Remove the apple crisp from the oven and let it rest for 5–10 minutes so the juices can settle.
Spoon the warm apple crisp into bowls and top with vanilla ice cream. For an extra touch, drizzle with caramel sauce before serving.