Set your oven to 350°F (175°C) so it’s nice and hot by the time you’re ready to bake. Lightly grease a 9x9-inch baking dish (or something close in size) with butter or nonstick spray, and set it aside.
Place the peeled, cored, and sliced apples in a large mixing bowl. Add the granulated sugar, light brown sugar, cinnamon, nutmeg, allspice, lemon juice, and cornstarch.
Toss everything together until the apple slices are evenly coated and the mixture looks well combined.
Transfer the filling to the prepared baking dish, spreading it out in an even layer.
In a separate bowl, combine the oats, flour, brown sugar, granulated sugar, cinnamon, and salt.
Add the cold, cubed butter. Using a pastry cutter or your fingertips, work the butter into the dry ingredients until the mixture forms coarse, crumbly pieces.
If you’re adding chopped pecans or walnuts, stir them in at this point.
Evenly distribute the crumble topping over the apple mixture. Try to cover the apples fully without pressing the topping down too much.
Place the dish in the center of your preheated oven. Bake for 35 to 40 minutes, or until the apples are soft and the topping is crisp and golden brown. You may see juices bubbling at the edges - this is a good sign!
Once baked, remove the crisp from the oven and let it cool for about 5 to 10 minutes.
Scoop portions of the warm crisp into bowls. Top each with a scoop of vanilla ice cream and, if desired, a drizzle of caramel sauce for extra indulgence. Enjoy!