Preheat oven to 180 degrees C
Grease and line with baking paper a 26cm spring form tin with baking paper
In a medium sized saucepan arrange the oranges and lemons so that they fit snugly. Fill with water to a level two thirds of the way up the sides of the fruit. Place over a medium heat and bring to the boil.
Keep an eye on it and after approximately 10 minutes of cooking, turn the lemons and oranges over. Continue cooking until the fruit has softened and the water has nearly evaporated.
Remove the saucepan from the heat and cut all the fruit into chunky pieces, removing any visible pips. Place the fruit into a food processor and puree whilst still hot
Add the eggs, sugar, almonds, baking powder, and vanilla to the pureed fruit and process for a minute until thick and smooth.
Pour into the lined tin, sprinkle with the sliced almonds and bake for 30 to 40 minutes. Test by pushing a skewer into the centre of the cake. When removed a wet crumbly mixture should remain on the skewer
To prepare the citrus syrup
In a saucepan over a medium to high heat combine the caster sugar and lemon juice, bring to the boil and simmer dissolving the sugar. Continue boiling to let the mixture reduce until syrupy in consistency.
With a skewer, pierce holes in the cake to allow the syrup to sink in. Pour half the syrup over the cake whilst still hot. Remove from the tin and serve with the remaining syrup and whipped cream.

To prepare the citrus syrup
In a saucepan over a medium to high heat combine the caster sugar and lemon juice, bring to the boil and simmer dissolving the sugar. Continue boiling to let the mixture reduce until syrupy in consistency.
With a skewer, pierce holes in the cake to allow the syrup to sink in. Pour half the syrup over the cake whilst still hot. Remove from the tin and serve with the remaining syrup and whipped cream.