1. Preheat the oven to 390°F (200°C). Line a baking sheet with baking parchment and set aside. Wash the rhubarb thoroughly and dry. Cut off the ends and then cut into 1.6 inches (4cm) pieces – if the rhubarb is very thick, cut those pieces lengthwise in half as well. Mix with the sugar and lemon zest and place on the prepared baking sheet. Cover with aluminum foil and roast for about 20 minutes until the rhubarb is cooked and softer. Remove from the oven and let cool down. Take about 3.5 oz. (100g) of the roasted rhubarb and the raspberries (optional) and puree in a mixer or with an immersion blender – this will be used in the cheesecake filling. Keep the rest of the rhubarb in the fridge until needed.
2. Add the biscoff cookies to a freezer bag and crush with a rolling pin until you get fine crumbs. Melt the butter and add together with the salt to the bag – mix until the crumbs are evenly moist. Line an 8 inches (20cm) springform tin with baking parchment and transfer the crumbs into the tin. Press to the bottom (with the back of a spoon) to create one even layer. Place in the fridge until needed.
3. Add the gelatin leaves to a small bowl with cold water and let soak for 5-7 minutes. Add the room temperature cream cheese, sour cream, sugar, and vanilla extract to a large bowl and mix until well combined. Set aside.
4. Squeeze the gelatin leaves to remove some of the water, add to a small pot and heat up until the gelatin has melted. Remove from the heat and add about 1-2 tablespoons of the cream cheese mix to the pot and combine with the gelatin. Pour this mixture back into the large bowl and mix well. Whisk the heavy cream until stiff peaks form. Add to the bowl and fold in. Add the pureed rhubarb and fold in – it is fine to see streaks. Pour the filling into the prepared springform tin, smooth out the top and place in the fridge for at least 4 hours or overnight.
5. Remove the cake from the tin and place on a serving plate. Decorate with the remaining roasted rhubarb and some chopped pistachios.