Preheat the oven to 180C fan. Line a 20cm square tin with baking paper.
Mix the flour, oats and brown sugar. Rub in the cold butter until the mixture forms clumps.
Press two-thirds of the mixture firmly into the tin. Bake 12 minutes until the surface looks dry.
Toss the apples with lemon juice, cornflour, cinnamon and cardamom. Spread over the warm base.
Stir the flaky salt into the caramel and drizzle over the apples. Scatter the remaining crumble mixture on top.
Bake 30-35 minutes until the topping is golden and the apple layer is bubbling.
Cool completely in the tin, chill 30 minutes, then cut into 16 neat squares.
Tap to reveal the secret seasoningGround cardamom – a small pinch gives the apple filling a gentle citrusy warmth that works beautifully with caramel and cinnamon.