Preheat the oven to 160C fan. Line a 20cm springform tin. Crush the biscuits, mix with melted butter and press into the tin. Chill.
Cook the apples with the sugar and 1 tablespoon of water for 5 minutes. Fold in the blackberries and cornflour, then cool.
Beat the cream cheese and sugar until smooth. Mix in the eggs one at a time, then the sour cream, vanilla and lemon zest.
Pour the filling over the base and spoon the cooled fruit across the top.
Rub the flour and butter into crumbs, stir in the brown sugar and oats. Scatter over the fruit.
Bake 50-55 minutes until the edge is set but the centre has a gentle wobble. Turn off the oven, open the door, cool inside 1 hour.
Chill at least 4 hours before removing from the tin and slicing.
Tap to reveal the secret seasoningLemon zest – it sharpens the berries and quietly lifts the rich cream-cheese filling without making the cheesecake taste lemony.