Wash the clementines and lemon, then put them in a large saucepan and cover with water. Bring to a boil, simmer for about an hour and a half, until very soft, then remove from the pan and leave to cool
While the fruit is simmering, blitz all the streusel ingredients in a food processor, then tip into a bowl and put in the fridge.
Heat the oven to 200C (180C fan)/gas 6, and grease a 24cm, loose-bottomed cake tin with butter. Sprinkle with the demerara sugar, tilting the tin so it coats all of the sides – there’s no need for baking paper, because the sugar will stop the cake from sticking.
Once the clementines and lemon are cool enough to handle, cut them open and remove and discard the pips. Add the clementines and lemon (including skin) to the same food processor you blitzed the streusel topping in (no need to wash it), and blitz on high until you have a smooth puree.
In a stand mixer, beat the eggs and brown sugar for a minute or two, until pale and fluffy, then add the ground almonds, citrus puree, baking powder and a pinch of salt, and mix again until light and smooth. Pour the mixture into the prepared tin, then sprinkle over the streusel topping. Scatter over the remaining tablespoon of demerara sugar and the roughly chopped pecans.
Bake in the middle of the oven for an hour, or until a skewer comes out clean, then leave to cool in the tin. Turn out and serve with a dollop of creme fraiche.