Heat the oven to 375°F (190°C). Butter six ramekins and arrange them on a rimmed baking sheet, leaving a little room between each.
Peel, core, and dice apples into small 1/2-inch pieces. Toss with sugar, cornstarch, lemon juice, and cinnamon until coated.
Divide apples evenly between ramekins, filling below the rim to leave space for topping and bubbling juices. Spoon any remaining coating over the fruit.
Mix topping flour, brown sugar, and salt. Cut cold butter into the dry mixture until it forms crumbs with some small clumps.
Divide topping between ramekins and scatter loosely. Avoid pressing a solid lid over the fruit, since steam needs a way to escape.
Bake 25–35 minutes, until the tops are golden and juices bubble around the edges. Pierce an apple piece in a central ramekin to check tenderness.
If ramekins brown unevenly, rotate the sheet near the end of baking. Remove only when the fruit beneath the topping is cooked.
Cool at least 20 minutes before serving on heatproof saucers. Warn guests that ramekins may remain hot even after the topping has cooled slightly.
