Wash the whole fruit and place in a large pot without peeling.
Cover with water and bring to the boil, boiling for two hours. Check the water level after an hour and top up if necessary.
Remove the fruit and leave to cool to room temperature.
When the fruits are cooled puree them until fine. This can be done ahead of time.
Pre-heat the oven to 190 degrees Celcius or 170 fan / 375 F / Gas mark 5.
Grease and flour a 20 cm baking round springform cake tin or two loaf tins.
Sift the almond flour, salt and baking powder in a bowl and set aside.
Place the eggs and sugar in the bowl of a stand mixer and beat until the eggs have increased in volume and are light in color, about 5 minutes.
Add the vanilla extract and citrus fruit puree and beat until combined.
Add the dry ingredients until all are combined.
Spoon the mixture into the cake pan and tap on the counter twice to remove any air bubbles.
Scatter over the slivered almonds and press into the surface lightly to ensure they stick.
Bake on a lower shelf for 50-60 minutes or until a skewer comes out clean. Check after 50 mins.
Loaves may require a shorter baking time so check after 40 mins.
If the cake seems to be browning too quickly cover with a sheet of parchment paper.
Baking the cake with the almonds may result in some sinking in the centre due to the weight of the nuts. You can serve it either decorated with fruit and a dusting of icing sugar, or turn it over and cover in the same way.