In a large mixing bowl, toss the cubed apples with the granulated sugar, cornstarch, cinnamon, nutmeg, allspice, and lemon juice until evenly coated. Spread the spiced apples into the bottom of a large casserole dish.
In the same large mixing bowl (no need to wash it!), combine the oats, flour, brown sugar, cinnamon, salt and stir until it is well combined. Pour in the melted butter and stir until the topping is evenly mixed and easily clumps together. Spread the prepared oat topping over top of the apples in the casserole dish, ensuring it is spread in a thick, even layer.
Bake for 45-50 minutes, or until the topping is golden brown and the apple filling is bubbling at the edges. Let the crisp rest for 10-15 minutes before serving warm, ideally with a scoop of vanilla ice cream on top.
Notes
Apples: The best types of apples to use are Honeycrisp, Granny Smith, Braeburn, Kanzi, or Gala. They can be left unpeeled for a more rustic crisp, or peeled for a smoother texture.
Cornstarch: Cornstarch is what thickens the filling, and bringing the filling to a full boil at the edges is required to activate the cornstarch and thicken the crisp properly.
Spices: You can swap out the individual spices for pumpkin pie spice or apple pie spice blends if that's what you have on hand.
Different Fruits: Swap the apples for blueberries, rhubarb, cherries, or strawberries for a different flavor fruit crisp.
Cut the apple cubes into uniform sizes. Randomly sized apple cubes cook at different rates and can make your apple crisp bake unevenly.
Store leftovers tightly wrapped with plastic wrap or in an airtight container in the refrigerator for up to 5 days. Reheat in the microwave or oven.