1. Grease a 25cm (10in) springform cake tin and line the base with baking paper. Pulse the biscuits in a food processor until crushed. Add the butter and process until it just clumps together. Press the crumb mixture into the base of the tin and two-thirds of the way up the sides. Refrigerate for 30 minutes.
2. Purée the berries and sugar in a food processor or blender and then pass through a sieve. Simmer in a small saucepan, stirring to dissolve the sugar for about 6 minutes or until thickened slightly. Set aside to cool.
3. Heat the oven to 170ºC (gas mark 3). Beat the cream cheese and caster sugar until its creamy and well combined. Add the whole eggs, egg yolks, sour cream, vanilla extract, lemon zest and lemon juice. Beat for 2-3 minutes, or until the mixture is light and fluffy.
4. Using a clean whisk and a clean bowl, beat the egg whites for 3-4 minutes, or until firm but soft peaks form. Using a large metal spoon, gently fold the egg whites into the cream cheese mixture.
5. Spoon the mixture into the prepared cheesecake base and smooth the top. Drizzle with the blueberry purée and then make swirl patterns with the blade of a knife.
6. Bake for 50 minutes to 1 hour, or until a pale golden colour and set in the centre. Switch off the oven and leave the cheesecake inside for 2 hours with the door ajar - this helps prevent it cracking. When cool, refrigerate for 6-12 hours before serving.
Now, as you can see, it didn’t quite turn out the way a cheesecake ideally should! I last made this in a conventional oven and it turned out beautifully. Yesterday I made it in a fan oven. I turned the temperature down to 160 degrees but I think it needed to be lower.
However odd it looked though, it tasted delicious. My pregnant sister in law managed 5 slices of it in one sitting!
Next time I make it, I’m going to turn the fan oven down to about 145/150 degrees so it doesn’t look quite so brown on top.