Put the clementines in a saucepan, cover with water and bring to the boil. Reduce the heat to low and cook gently for about 40 minutes until very soft. Drain and leave until cool enough to handle. Quarter and discard any pips. Blend to a purée in a food processor. Turn into a large bowl and beat in the egg yolks, 150 g (5 oz) of the caster sugar, ground almonds and baking powder.
Whisk the egg whites in a large clean bowl with a hand-held electric whisk until peaking. Gradually whisk in the remaining caster sugar, a spoonful at a time. Stir a quarter of the mixture into the clementine mixture using a large metal spoon. Add the remaining egg whites and stir gently to mix.
Spoon the mixture into a greased and base-lined 22–23 cm (8½–9 inch) spring-form cake tin (see
Leave to cool in the tin, then turn out. Scatter with the summer fruits, dust with sifted icing sugar and serve with Ginger Marmalade Cream, if liked (see below).