For the base, butter a 20 cm spring form pan with melted butter. Whizz the base ingredients in a blender until they are like fine breadcrumbs.
Pour this crumb mixture into a bowl and add the melted butter. Mix everything together then tip into the spring form pan.
Press it down so it is level and compact with the back of a large spoon then put the tin in the fridge for a good 30 mins to firm up.
Preheat oven to 200C/400F and line a Swiss roll baking tray with parchment. Place the blueberries on the tray and sprinkle with sugar.
Roast for 15-20 minutes until the juices run out. Soak gelatine sheets in cold water for 5 minutes and then squeeze out excess water.
Puree the hot blueberries and add the squeezed gelatine sheets into it and stir to melt. Then add the cream cheese, ricotta, sugar, vanilla and lemon zest. Pour into the prepared tin over the base and allow to set for 4-6 hours or overnight.