Heat the oven to 375°F. Lightly butter an 8-inch square dish and set it on a rimmed sheet pan.
Toss apples with granulated sugar, cornstarch, lemon juice and 1/2 teaspoon cinnamon until no dry starch remains.
Mix flour, oats, brown sugar, salt and the remaining cinnamon. Rub in cold butter until the mixture holds together in pea- to walnut-size clumps.
Spread the apples evenly, including any juice in the bowl. Scatter the crumble loosely over the top without pressing it flat.
Bake 40 to 50 minutes, until the topping is golden and the filling bubbles slowly near the center, not just at the edges.
Cool at least 20 minutes before serving so the apple juices thicken around the fruit.