Preheat the oven to gas 4, 180°C, fan 160°C.
Melt the butter in a large saucepan. Grease the base and sides of a round 23cm (9 in) springform tin, then line the base with nonstick baking paper. Stir the crushed biscuits into the butter until they are completely combined – the mixture should have the consistency of wet sand. Press into the tin and smooth with the back of a spoon. Bake for 15 minutes until golden, then remove from the oven and leave to cool.
Meanwhile, beat the soft cheese, mascarpone, vanilla and lemon zest with an electric whisk until well combined, then add the sugar and continue beating until smooth. Add the eggs and yolks one at a time, beating after each addition until smooth and creamy.
Spoon the filling onto the biscuit base in a smooth, even layer. Wrap the base and sides of the tin tightly with a wide sheet of foil until watertight and place in a large roasting tin.
Pour enough boiling water into the roasting tin to come halfway up the sides of the cheesecake tin. Bake for 10 minutes at gas 4, 180°C, fan 160°C, then turn the oven down to gas 1, 140°C, fan 120°C and bake for a further 40-50 minutes or until the cheesecake is set, with a slight wobble in the centre. Let the cheesecake cool in the oven, with the door ajar. When cold, remove and chill for 4 hours.
For the topping, put the blueberries, sugar and lemon juice into a pan. Heat gently for 10 minutes until the sugar has dissolved and the berries have released their juice. Mix the arrowroot with 1 tsp water, add to the pan and stir until thick and shiny. Remove from the heat and set aside to cool.
Half an hour before serving, remove the cheesecake from the fridge. Slide a palette knife around the edge of the cake to loosen it, then carefully remove it from the tin. Transfer the cheesecake to a plate then serve with the topping and some pouring cream, if liked.
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