Prep. Heat the oven to 375°F (190°C) and grease an 8×8 or 9×9-inch baking dish.
Make the filling. Toss the sliced apples with the sugar, flour, cinnamon, lemon juice, and cider; spread evenly in the dish.
Make the topping. Whisk the flour, brown sugar, cinnamon, and salt, then cut in the cold butter until it resembles coarse crumbs with a few pea-sized pieces.
Assemble. Scatter the topping evenly over the apples without pressing it down.
Bake. 45–50 minutes, until the filling bubbles at the edges and the topping is deep golden.
Rest. Cool 15–20 minutes so the filling sets before you scoop.