Start by making the base: Pre-heat the oven to 180 C / 160 C fan. Place the hazelnuts on a baking tray and place into the oven. Bake for 8 - 10 minutes, until the hazelnuts smell toasted and are starting to turn golden. Add to a food processor and blitz until a fine crumb is formed- be careful not to over blitz though as you don’t want to make nut-butter.
Melt the butter over a medium heat until slightly brown and nutty smelling then tip into the food processor and pulse to combine.
Press the toasted crust into a 23cm spring form tin lined with a circle of greaseproof paper and smooth out with your hands or the back of a glass, then bake for 8 minutes.
Remove the crust from the oven and allow to cool whilst you get on with making the cheesecake filling.
Make the filling: Using an electric hand whisk the sour cream, cream cheese and powdered sweetener together. Add the eggs and egg yolks and mix until smooth, then stir through the lemon juice and vanilla.
Wrap two layers of foil securely around the outside of the cake tin with the base in - covering the bottom and up the sides, to prevent any water seeping in. Put the tin into a deep oven tray or dish, to act as a water bath, and boil the kettle.
Pour the cheesecake filling onto the base in the tin and cover the top of the tin loosely with foil, then put the whole thing into the oven. Pour boiling water into the water bath, so that it comes half-way up the cheesecake. Bake the cheesecake for 1hr 30 minutes, until just set. Switch off the oven and leave the cheesecake in there for 1 hour, so it cools slowly and is less likely to crack. Then remove the cake tin from the water bath and leave it to cool completely, in its tin, before putting in the fridge for at least 4 hours, or overnight.
To make the blueberry compote: Place blueberries, sweetener and lemon juice in a small saucepan and bring to a simmer.
Simmer on a low heat for 5 - 8 minutes or until the blueberries have started to break down then
remove the pan from the heat and allow to cool slightly.
To assemble the cheesecake: Run a hot pallete knife around the edge of the cheesecake to loosen it from the edge of the tin. Carefully transfer it to a serving plate, and decorate with the compote then serve, slicing with a hot knife to get a clean slice.