Brush the spring form tin with a little oil, line with cling film.
Cut the butter into pieces, roughly chop the chocolate. Place both in a thin-sided bowl and suspend over the gently simmering bain-marie, ensuring that the bowl does not touch the water. Melt the butter and chocolate and stir until smooth.
Blitz the biscuits in a food processor or crush finely in a plastic bag using a rolling pin. Add the biscuits to the chocolate and butter mixture and mix.
Place the spring form tin on a cake plate, spread the biscuit mixture over the base, press down firmly with the base of a glass and chill for approx. 15 mins.
Rinse the clementines in hot water and pat dry. Grate the zest of the clementines and squeeze out approx. 100 ml of juice. Combine with the quark and gelatine.
Beat the egg whites with the salt until stiff, add the sugar, continue beating for a moment until the egg whites turn glossy. Carefully fold the egg whites into the quark mixture and spread over the biscuit base. Cover the cheesecake and leave to set in the fridge for approx. 4 hrs.
Rinse the clementines in hot water, pat dry and cut into slices approx. 4 mm thick.
Bring the water to the boil with the sugar. Add the clementines, reduce the heat and simmer for approx. 10 mins. Remove the clementines from the syrup, drain and place on a rack to cool. Repeat this step once more.
Remove the tin. Top the cheesecake with the clementines. Roughly chop the pistachios and scatter them over the top.