Preheat the oven to 350°F.
Grease a 9x13-inch baking dish with butter or nonstick spray.
Place the sliced apples in a large mixing bowl.
Drizzle the apples with lemon juice and toss to coat.
Add the granulated sugar, cinnamon, and cornstarch to the apples and mix well.
Transfer the apple mixture to the prepared baking dish, spreading it evenly.
In a separate bowl, combine the oats, flour, brown sugar, granulated sugar, cinnamon, and salt.
Add the cold butter cubes to the dry ingredients.
Use a pastry cutter, fork, or your fingers to mix until the topping forms coarse crumbs.
Evenly sprinkle the topping over the apples, covering them completely.
Bake for 40-45 minutes, or until the topping is golden brown and the apples are bubbling.
Remove from the oven and let it cool for at least 10 minutes before serving.
Serve warm with vanilla ice cream or whipped cream if desired.