Preheat your oven to 160°C (320°F). Line an 8x8 inch baking pan with baking paper.

In a mixing bowl, crush the digestive biscuits until they resemble fine crumbs. Melt the unsalted butter and combine it with the crumbs. Press this mixture into the base of the prepared pan and chill for 15 minutes.

In a large bowl, beat the full-fat cream cheese with granulated sugar until smooth. Add salt and vanilla extract, then incorporate the eggs one at a time, blending until combined. Pour the cheesecake filling over the chilled crust.

Toss the diced Granny Smith apples with sugar, lemon zest, cinnamon, and nutmeg. Allow standing for 5 minutes, then layer on top of the cheesecake filling.

Combine chilled unsalted butter with plain flour and demerara sugar. Mix until it resembles coarse crumbs, then sprinkle over the apple layer.

Bake for 45-50 minutes, until the crumble is golden and the cheesecake is set with a slight jiggle. Let the pan cool completely.

Refrigerate for at least 3 hours to set. Before serving, drizzle with caramel sauce and slice into squares.