Preheat oven to 350°. In a medium bowl, toss apples, cornstarch, lemon juice, granulated sugar, nutmeg, and 1 teaspoon cinnamon.
In a small bowl, toss oats, brown sugar, flour, salt, and remaining 1 teaspoon cinnamon. Add butter and stir until a crumbly mixture forms resembling wet sand.
Arrange apple mixture in a 2-quart ceramic baking dish. Top with oat mixture.
Bake until apples are soft, topping is crisp and golden brown, and juices are bubbling around edges of dish, 55 to 70 minutes. Serve warm with ice cream (if using).
