Heat the oven to 350°F (175°C). Lightly butter the baking dish and set it on a rimmed sheet to catch bubbling fruit juices.
Peel, core, and cut apples into even 1/4-inch slices. Toss with sugar, lemon juice, cornstarch, and cinnamon until the fruit is evenly coated.
Spread the apples in the dish, including any coating left in the bowl. Avoid a tall central mound so the fruit cooks at the same rate.
Mix topping flour, oats, brown sugar, cinnamon, and salt. Work in cold butter with a pastry blender until rough clumps form.
Scatter topping evenly over the apples without pressing it into a solid lid. Leave small openings where steam and fruit juices can escape.
Bake 40–50 minutes until topping is golden and juices bubble near the center. Cover loosely with foil if the top darkens before the apples soften.
Pierce a central apple slice with a small knife; it should be tender. Extend baking briefly if the fruit still feels firm under the topping.
Rest 20 minutes before spooning into bowls. Serve warm, letting the juices thicken slightly rather than portioning immediately from the oven.
