Preheat oven to 350°F (175°C) and line a baking dish with parchment paper.
Make the crumble: In a medium bowl, mix together the flour, sugar, cinnamon, and salt. Add the butter and rub it between your fingertips until the mixture resembles coarse crumbs.
Prepare the apples: Peel and slice the apples evenly into wedges. Arrange them in the prepared baking dish in a single, even layer.
Add the topping: Sprinkle the crumble mixture evenly over the apples, covering them completely.
Bake for 30 minutes or until the topping is golden and the apples are bubbling around the edges.
Cool slightly before serving to let the juices thicken.
Serve warm — plain, or with a scoop of ice cream or a spoonful of yogurt.