Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper. Spread the frozen apple slices evenly over the sheet, then sprinkle with cinnamon, sugar, and salt. Bake for 10-12 minutes until the edges are golden and caramelized, then remove and let cool slightly.
While the apples cool, prepare the crumble topping. In a mixing bowl, combine flour, brown sugar, and cold cubed butter. Use a pastry cutter or your fingers to work the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces.
Spoon the caramelized apple slices into a baking dish, spreading them out evenly. Cover the apples with the prepared crumble mixture, pressing lightly to adhere.
Bake the assembled crumble at 375°F (190°C) for 20-25 minutes, or until the topping is golden brown and the mixture is bubbling around the edges.
Remove from oven and let rest for 5 minutes. Serve warm, topped with a scoop of vanilla ice cream. Observe the contrast between the crispy topping and soft, jammy apples.
