Separate the egg whites from the egg yolks; beat the egg whites with an electric whisk and, when they begin to harden, add 30 gr of sugar.
Continue to mount until they are firmly in place.
Work the egg yolks with the remaining sugar, half a teaspoon of grated orange zest and the flour with an electric whisk. When a smooth cream has been created, add the whipped egg whites and mix gently with a spatula.
Transfer the mixture to a pan of about 25 cm in diameter lined with oven paper and bake in a preheated oven at 170°C for about 15 minutes.
Remove the sponge cake from the oven and cool it quickly with the help of the Freddy blast chiller with the blast chilling function for 10 minutes.
Meanwhile, squeeze the juice of two oranges, heat it in a saucepan and dissolve 15g of isinglass previously soaked in cold water.
Whip the cream with the icing sugar, stir in the Greek yogurt and finally add the orange juice with the isinglass and 1 teaspoon of grated orange zest.
Remove the sponge cake from Freddy and remove it from the pan, pour the cream and yogurt mixture into the same pan, place the sponge cake disc on top and place everything in Freddy to firm with the blast chilling function for approximately 1 hour.
Squeeze the juice of 2 other oranges, heat it with 1 tablespoon of rum and dissolve the remaining isinglass soaked in cold water in it.
When the mousse cake is firmed, remove from the cake pan so that the sponge cake is underneath and the mousse on top, pour the rhum - flavoured orange juice over it and decorate with slices of red grapefruit and orange.