Preheat the oven to 160°C fan (180°C conventional) and grease a 20–23cm springform tin.
Mix the crushed digestive biscuits with the melted butter and press firmly into the base of the tin.
Bake the base for 10 minutes, then allow it to cool slightly.
Beat the cream cheese until smooth before mixing in the caster sugar.
Add the eggs one at a time, then stir in the sour cream, vanilla extract, plain flour, and lemon zest until just combined.
Pour the filling over the cooled biscuit base and smooth the surface.
Rub the cold butter into the plain flour until it resembles breadcrumbs, then mix in the caster sugar and rolled oats.
Sprinkle the crumble mixture evenly over the cheesecake.
Bake for 50–60 minutes until the edges are set and the centre has a slight wobble.
Turn off the oven and leave the cheesecake inside with the door slightly open for 1 hour.
Chill for at least 2 hours, or overnight for the best texture.
If desired, top with berry compote before serving.