Preheat the oven to 190°C, fan 170°C, gas 5. Grease a 22cm springform cake tin and line with baking parchment.
Wash the clementines and put in a pan. Cover with water and bring to the boil, then reduce to a simmer and cover. Cook for 1 hour, adding water to make sure the pan doesn't go dry. Drain and allow to cool.
Quarter the clementines and discard any pips. Place in a blender (with the skin intact) and purée until smooth.
Whisk the eggs and sugar with an electric hand whisk in a large bowl until pale and fluffy; about 6-7 minutes. Fold in the clementine purée, ginger, mixed spice, almonds and baking powder.
Pour the mixture into the prepared tin and cook in the centre of the oven for 1-1½ hours. Insert a skewer into the centre of the sponge and if it comes out clean the cake is cooked. Allow to cool in the tin before removing. Dust with caster sugar and serve with crème fraîche or whipped double cream.