Place everything in a saucepan and cook over medium heat until it thickens. It should look like a loose jam and you should be able to see the bottom of the pan when you stir. With your spatula, flatten out the blueberries as they cook so that some of them disintegrate a bit.

Remove the sauce from the pan and let it cool down completely. Divide it in two: one half will go to the filling and the other half on top.

Process the cookies until fine.

Add the salt and butter and mix.

Cut baking paper to the size of the base of the pan and also the sides. Stick them in place with vegetable oil that you can spread with a piece of kitchen paper.

Press the base mix onto the base of the pan using the base of a glass and then with your fingers around the edges. Take the pan to the freezer for at least 15 minutes.

Beat the cream cheese with the sugar until it's really soft and homogeneous. Every once in a while stop and scrape down the sides and bottom of the bowl to make sure that everything gets beaten.

Add the lemon zest and sour cream, salt and beat again.

Reduce the speed to a minimum and add one egg at a time. You know that you can add the next one when the previous one is incorporated.

Add the lemon juice and vanilla extract and mix them in with a spatula.

Pour half of the mix onto the base and add dollops of the blueberry sauce. Use a knife or skewer to marble it. Be careful not to disturb the base below. Repeat with the other half of the filling.

Bake the cheesecake in a preheated oven at 180°C / 350°F. When you preheat the oven you want to have a baking pan with water at the bottom layer of your oven so the water preheats, too. If you forget to do it, you can add boiling water. The cheesecake goes on the level above it (not in the water). In this way, the cheesecake still cooks gently but you remove the risk of the water going into the cake.

As soon as it goes into the oven, reduce the heat to 150°C /300°F.

Bake the cheesecake for 1h-1h30min or until you see it starting to get golden on top and it jiggles like gelatin if you shake it slightly. Halfway through the cooking process, check that the water level hasn't dropped too much. If it has, fill it up with boiling water.

Wait for it to cool down completely and place it in the fridge overnight or for at least 8 hours.

Unmould the cheesecake by opening the springform pan and peeling off the paper on the sides. To remove it from the base I use a cake lifter.

Spread the other half of the blueberry sauce on top.

Beat the cream and icing sugar until it becomes fluffy and keeps its shape if you move it around with a spatula or whisk.

Use a pastry bag fitted with a star nozzle to make 4 rosettes at the edge as if they were 12, 3, 6 and 9 on a clock. Then between them make more rosettes depending on the amount of portions you'll be splitting the cheesecake in. In my case, it was 12 portions.

Garnish with lemon and lemon zest.