Set the oven to 350°F or 175°C and grease a 9-inch baking dish. A shallow dish allows the topping to brown more evenly and gives each serving a generous layer of crumble.
Add the apple slices to a mixing bowl with sugar, lemon juice, cinnamon, vanilla, and cornstarch. Stir carefully until the cornstarch disappears and the fruit looks lightly glossy.
Spread the seasoned apples across the baking dish. Arrange the slices evenly so the filling cooks at the same rate from the center to the edges.
Combine flour, brown sugar, cinnamon, nutmeg, and salt in another bowl. Add the cold butter and rub it into the dry ingredients with your fingertips until irregular crumbs form.
Scatter the crumble mixture loosely over the fruit. Some larger clumps are helpful because they create buttery, crisp pockets once baked.
Bake for 42 to 47 minutes, until the topping turns golden and the fruit juices begin bubbling. Allow the crumble to cool for at least 10 minutes before spooning it into bowls.
